Current Directions in Psychological
Science
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Mutually Responsive Orientation Between Mothers and Their Young Children: A Context for the Early
Development of Conscience
Grazyna Kochanska
Current Directions in Psychological Science 2002 11: 191
DOI: 10.1111/1467-8721.00198
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CURRENT DIRECTIONS IN PSYCHOLOGICAL SCIENCE
These basic questions are central to
any evolutionary approach. Psychologists who do not like the simplicity of the answers currently
coming out of evolutionary psychology should make an effort to
improve them, to broaden its intellectual horizon, because all of psychology would stand to gain from
a more enlightened evolutionary
psychology.
Recommended Reading
de Waal, F.B.M. (1999). The end of
nature versus nurture. Scientific
American, 281, 94–99.
de Waal, F.B.M. (2001). The ape and
the sushi master: Cultural reflections
by a primatologist. New York: Basic
Books.
Mayr, E. (2001). What evolution is.
New York: Basic Books.
Zimmer, C. (2001). Evolution: The triumph of an idea. New York: Harper
Collins.
Acknowledgments— I thank Allison
Berger and Virginia Holt for providing
the transcript of my 2001 Focus on Science Plenary Address, which was presented at the annual meeting of the
American Psychological Association in
San Francisco and was on the topic of this
essay. I am also grateful to Mauricio Papini and Scott Lilienfeld for comments on
previous versions of the manuscript.
more: Johns Hopkins University Press. (Original work published 1982)
de Waal, F.B.M. (2000, April 2). Survival of the
rapist [Review of the book A natural history of
rape: Biological bases of sexual coercion]. New York
Times Book Review, pp. 24–25.
Dobzhansky, T. (1973). Nothing in biology makes
sense except in the light of evolution. American
Biology Teacher, 35, 125–129.
Gauthier, I., & Tarr, M.J. (1997). Becoming a “Greeble” expert: Exploring mechanisms for face
recognition. Vision Research, 37, 1673–1682.
Notes
1. Address correspondence to Frans
B.M. de Waal, Living Links, Yerkes Primate Research Center, Emory University, 954 N. Gatewood Rd., Atlanta, GA
30322.
2. Theory of mind means that one
understands the mental states of others
(a capacity that may be limited to humans and apes).
References
Daly, M., & Wilson, M. (1988). Homicide . Hawthorne, NY: Aldine de Gruyter.
de Waal, F.B.M. (1996). Good natured: The origins of
right and wrong in humans and other animals.
Cambridge, MA: Harvard University Press.
de Waal, F.B.M. (1998). Chimpanzee politics. Balti-
McDougall, W. (1908). An introduction to social psychology. New York: Putnam.
Muscarella, F., & Cunningham, M.R. (1996). The
evolutionary significance and social perception of male pattern baldness and facial hair.
Ethology & Sociobiology, 17, 99–117.
Panksepp, J., & Panksepp, J.B. (2000). The seven
sins of evolutionary psychology. Evolution and
Cognition, 6, 108–131.
Staats, A.W. (1991). Unified positivism and unification psychology: Fad or new field? American
Psychologist, 46, 899–912.
Thornhill, R., & Palmer, C.T. (2000). A natural history of rape: Biological bases of sexual coercion.
Cambridge, MA: MIT Press.
Tooby, J., & Cosmides, L. (1992). The psychological foundations of culture. In J. Barkow, L.
Cosmides, & J. Tooby (Eds.), The adapted mind:
Evolutionary psychology and the generation of culture (pp. 19–136). New York: Oxford University Press.
Williams, G. (1966). Adaptation and natural selection.
Princeton, NJ: Princeton University Press.
Wilson, E.O. (1998). Consilience: The unity of knowledge. New York: Knopf.
Mutually Responsive Orientation
Between Mothers and Their Young
Children: A Context for the Early
Development of Conscience
dinal beneficial effects of MRO
for early development of conscience have been replicated
across studies, for a broad range
of developmental periods from
infancy through early school
age, and using a wide variety of
behavioral, emotional, and cognitive measures of conscience in
the laboratory, at home, and in
school. These findings highlight
the importance of the early parent-child relationship for subsequent moral development.
Grazyna Kochanska1
Department of Psychology, University of Iowa, Iowa City, Iowa
Abstract
Some parent-child dyads establish a mutually responsive
orientation (MRO), a relationship that is close, mutually
binding, cooperative, and affectively positive. Such relationships have two main
characteristics—mutual responsiveness and shared positive affect—and they foster the
191
development of conscience in
young children. Children growing up with parents who are responsive to their needs and
whose interactions are infused
with happy emotions adopt a
willing, responsive stance toward parental influence and become eager to embrace parental
values and standards for behavior. The concurrent and longitu-
Keywords
relationships; mutuality; conscience
How do young children become
aware of rules, values, and standards
of behavior accepted within their
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VOLUME 11, NUMBER 6, DECEMBER 2002
families and cultures? How do they
gradually come to internalize those
values and make them their own?
Why do some children adopt societal norms wholeheartedly and with
ease, and become conscientious citizens, whereas others do not?
The emergence of an individual
conscience, a reliable internal guidance system that regulates conduct
without the need for external control, is the endpoint of the process
of integrating a child into a broader
network of values. How this process works continues to be debated
as one of the perennial and central
issues in human socialization
(Grusec, 1997).
Research on conscience was once
dominated by a cognitive approach,
focused on children’s abstract understanding of societal rules, measured
by their ability to reason about hypothetical moral dilemmas. Moral development was seen as a product of
cognitive maturation, aided by peer
interactions, but fundamentally unrelated to parental influence. In contrast, other theories acknowledged
parental contributions. Parents and
other socializing agents were seen as
critical in several versions of learning
theory. Those approaches emphasized the importance of parental discipline and modeling as instruments
that modify and shape children’s behavior. Somewhat later, attributional
theories underscored the importance
of children’s perceptions of parental
discipline, and revealed surprising,
often paradoxical effects of salient
parental rewards and punishments.
More recently, many scholars
have come to appreciate an approach grounded in psychoanalytic and neo-psychoanalytic theories. Although Freud’s views on
the early development of conscience as linked to the Oedipus or
Electra complex have long been
discarded, his general emphasis on
the role of early emotions and early
relationships in emerging morality
has proven insightful. That approach has been strongly reinvigo-
rated and modernized by John
Bowlby and the burgeoning research on attachment. From that
perspective, moral emotions, moral
conduct, and moral thought are all
components of an internal guidance system, or conscience, whose
foundations are established in
early childhood in the context of
socialization in the family. The
early parent-child relationship,
which encompasses but is not limited to control and discipline, can
substantially foster or undermine
that process (Emde, Biringen, Clyman, & Oppenheim, 1991).
THE RELATIONSHIP
PERSPECTIVE: MUTUALLY
RESPONSIVE ORIENTATION
In 1951, Robert Sears argued for
a shift in psychological research
from studying individuals to
studying dyads. Over the past two
or three decades, the science of relationships has blossomed in personality, social, and developmental
psychology (Collins & Laursen, 1999;
Reis, Collins, & Berscheid, 2000). Several scholars have proposed that
when relationship partners—whether
two adults or a parent and a child—
are responsive and attuned to each
other, are mutually supportive,
and enjoy being together, they
form an internal model of their relationship as a cooperative enterprise, and develop an eager, receptive stance toward each other’s
influence and a compelling sense
of obligation to willingly comply
with the other. For example, Clark
(1984) referred to “communal relationships” in adults as contexts in
which the partners are invested in
each other’s well-being, are empathic and responsive to each
other, and experience an internal
sense of mutual obligation.
In developmental research, those
resurging perspectives afford a productive vantage point for exploring
social development. Socialization is
seen as a process jointly constructed
by parents and children over time
(Collins & Laursen, 1999; Collins,
Maccoby, Steinberg, Hetherington,
& Bornstein, 2000; Maccoby, 1999;
Reis et al., 2000). Maccoby (1999)
referred to parent-child mutuality
as a positive socialization force that
engenders a spirit of cooperation in
the child. Attachment scholars believe that children raised in a loving,
responsive manner become eager
to cooperate with their caregivers
and to embrace their values.
To describe such relationships
between parents and children, my
colleagues and I have proposed a
construct of mutually responsive orientation (MRO). MRO is a positive,
close, mutually binding, and cooperative relationship, which encompasses two components: responsiveness and shared positive affect .
Responsiveness refers to the parent’s and the child’s willing, sensitive, supportive, and developmentally appropriate response to one
another’s signals of distress, unhappiness, needs, bids for attention, or attempts to exert influence.
Shared positive affect refers to the
“good times” shared by the parent
and the child—pleasurable, harmonious, smoothly flowing interactions infused with positive emotions experienced by both.
We further proposed that children who grow up in mutually responsive dyads, compared with
those who do not, become more eager to embrace their parents’ values and more likely to develop a
strong conscience. Their eager
stance to embrace parental values
reflects an internal sense of obligation to respond positively to parental influence, and emerges from a
history of mutually gratifying, mutually accommodating experiences.
A child who has developed a mutually responsive relationship with
the parent comes to trust the parent and to expect that the parent
will be responsive and supportive;
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193
CURRENT DIRECTIONS IN PSYCHOLOGICAL SCIENCE
at the same time, the child comes to
feel motivated to cooperate willingly with the parent, to embrace
the parent’s values, and to adopt
parental standards for behavior
and make them his or her own. In
this view, the parent-child relationship influences the child’s conscience mainly through a gradually
evolving shared working model of
the relationship as a mutually cooperative enterprise rather than
through the cumulative history of
parental discipline as the instrument of behavior modification.
MOTHER-CHILD MRO AND
CHILDREN’S CONSCIENCE:
EMPIRICAL EVIDENCE
In two large studies, we measured the qualities of the motherchild relationship and the child’s
emerging conscience for more than
200 mother-child dyads. To assess
the strength of MRO for the individual dyads, we observed the
mothers and children interacting in
multiple lengthy, naturalistic yet
carefully scripted contexts at home
and in the laboratory. The situations we observed included caregiving routines, preparing and eating meals, playing, relaxing, and
doing household chores. We coded
each mother’s responsiveness to her
child’s numerous signals of needs,
signs of physical or emotional distress or discomfort, bids for attention, and social overtures. We also
assessed shared positive affect by
coding the flow of emotion expression for both the mother and the
child over the course of each interaction, focusing particularly on the
times when they both displayed
positive emotion. We obtained
these measures repeatedly, following the same families over a period
of several years.
In the individual dyads, the degree of MRO was significantly con-
sistent across separate sessions
close in time, and significantly stable over several years. This indicates that our observational markers captured a robust quality of the
relationships that unfolded along a
fairly stable dyadic trajectory.
Using a broad variety of laboratory paradigms, we also observed
rich manifestations of the young
children’s conscience: moral emotions, moral conduct, and moral
cognition. These assessments took
place at many points in the children’s development—starting in their
2nd year and continuing until early
school age. The children’s moral
emotions, including guilt, discomfort, concern, and empathy, were
observed when they were led to
believe that they had violated a
standard of conduct, or when they
witnessed others’ distress. While
they were unsupervised, either
alone or with peers, their moral
conduct was assessed in many
types of situations in which they
faced strong temptations to break
various rules and were coaxed to
violate standards of behavior.
Their moral cognition was measured by presenting them with
age-appropriate, hypothetical
moral dilemmas and asking them
to express their thoughts and feelings about rules and transgressions, and consider moral decisions. We also asked their mothers
and teachers to evaluate the children’s moral emotions and conduct displayed in environments
outside the laboratory—at home
and at school.
Both studies supported the view
that children who grow up in a
context of a highly mutually responsive relationship with their
mothers develop strong consciences
(Kochanska, 1997; Kochanska, Forman, & Coy, 1999; Kochanska &
Murray, 2000). The strength of the
replicated findings was striking,
given the broad range of the children’s ages and the wide variety of
conscience measures used.
In both studies, the links between MRO and the development
of conscience were both concurrent
and longitudinal. The concurrent
links were found for both toddlers
and preschoolers. The longitudinal
findings were robust: MRO in infancy predicted conscience development in the 2nd year, and MRO
in toddlerhood predicted children’s conscience at preschool age
and again at early school age. The
history of MRO in the first 2 years
predicted conscience at age 5. In
short, the beneficial effect of MRO
on the development of conscience
was evident across diverse measures
of conscience involving emotions,
conduct, and cognition. It was also
evident whether conscience was
assessed by observations in the laboratory or reports from mothers
and teachers. These results have
been replicated by other researchers (Laible & Thompson, 2000).
HOW DOES MRO EXERT
ITS IMPACT?
What causal mechanisms may
be responsible for these well-established empirical findings? Using statistical approaches (sequences of
multiple regressions, as well as
structural equations modeling, or
SEM) to analyze the causal factors
that accounted for the associations
in our data, we determined that
MRO exerts its influence through
at least two mechanisms.
The first mechanism involves
promoting the child’s positive
mood. Early MRO between the
parent and the child contributes to
the child’s positive, happy disposition, and that, in turn, increases his
or her broad eagerness to behave
prosocially. This finding is consistent with a large body of research
in social and developmental psychology (Eisenberg & Fabes, 1998).
Adults and children who are in a
positive mood have often been
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VOLUME 11, NUMBER 6, DECEMBER 2002
found to be more prosocial, altruistic, cooperative, rule abiding, and
socially responsive than those who
are in neutral or negative moods.
The second mechanism involves promoting the child’s responsive stance toward parental
influence. We have found that in
playlike teaching situations, children in mutually responsive relationships are attuned to their
mothers and eagerly follow their
lead (Forman & Kochanska, 2001;
Kochanska et al., 1999). In discipline situations, they show what we
called committed compliance—willing, eager, wholehearted cooperation with the parent (Kochanska,
Coy, & Murray, 2001). Such a generalized responsive stance may be
an intermediate step between simple cooperation with the parent
and genuine internalization of parental rules, evident even in the
parent’s absence. We believe it reflects the child’s emerging working
model of a cooperative, reciprocal,
mutually accommodating relationship in which partners naturally do
things for one another without abrogating their autonomy.
FUTURE RESEARCH
DIRECTIONS
MRO and Qualities
of Individuals
It takes two to develop dyadic
MRO. Although the relationship between a parent and child—like any
relationship—is more than a simple sum of their characteristics,
those characteristics may nevertheless foster or impede the formation
of MRO. Recent advances in research on the role of genetics in behavior and on the biological foundations of children’s temperament
are beginning to be reflected in scientific work in what has been traditionally conceived as the domain of
relationships. For example, Deater-
Deckard and O’Connor (2000),
studying identical and fraternal
twins, and biological and adoptive
siblings, found that parent-child
MRO was driven, in part, by the
child’s genetically based qualities.
In addition, a child’s biologically
based traits, such as being prone to
anger or joy, or being hard or easy
to soothe, may facilitate or undermine the evolution of the child’s relationships within particular dyads.
Being responsive to and having enjoyable interactions with a child
may be more challenging if the
child is temperamentally difficult
than if he or she is easygoing and
mellow.
Mothers’ traits, some also biologically based, may be important
as well. We have found that the
more empathic mothers are, the
better able they are to form MRO
with their children (Kochanska,
1997). A large body of research indicates that depression and high
levels of negative emotion in mothers reduce their responsiveness
and positive behavior when interacting with their young children.
More complex interplay between biological and relationship
factors also deserves future research attention. Our findings indicate that MRO may be particularly
beneficial for children with certain
temperaments, particularly fearless, thrill-seeking children whose
behavior is not easily modified by
actual or anticipated punishments
and threats. Other interactions between temperament and relationships are also possible.
MRO as a Developmentally
Changing System
A mutually responsive relationship between a parent and an infant differs from a mutually responsive relationship between a
parent and a preschooler, or between a parent and an adolescent.
The contexts and currency of par-
ent-child interactions change. In infancy, those contexts include mostly
the contexts of caregiving, play, and
daily routines, and the currency of
exchange is often nonverbal. Gradually, the contexts expand to include parent-child discussions of
events and ideas, and the exchanges
are increasingly verbal (Laible &
Thompson, 2000). The child’s and
the parent’s relative contributions
to the relationship change over time,
and so do their cognitive representations, perceptions, and expectations of the relationship and of each
other. Psychologists’ understanding
of the child’s side of MRO lags considerably behind their understanding of the parent’s side of MRO. How
MRO can be assessed in a manner
that is developmentally sensitive
and yet captures stable qualities of
the parent-child dyad over time is
one of the future challenges.
MRO and Internal
Representations
In research to date, MRO has been
inferred from parents’ and children’s observed behavior and affect during interactions. This outer
layer, however, only partially captures the essence of a relationship.
Scholars studying relationships
have adopted Bowlby’s premise
that, over time, the parent and the
child gradually form inner representations, or internal working
models, of their relationship (Collins & Laursen, 1999). Those evolving models include generalized
memories of each other’s behavior,
implicit beliefs and feelings about
each other and the relationship,
and a sense of what the relationship is like and what to expect
from one another. Those generalized products of an individual’s experience serve to organize and bias
his or her future information processing, behavior, and emotions. In
the case of MRO, the parent’s and
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CURRENT DIRECTIONS IN PSYCHOLOGICAL SCIENCE
child’s internal models entail mutual cooperation and implicit reciprocity, and the child’s internal
model is thought to underlie his or
her willingness to embrace parental rules. Those inner representations, however, are difficult to access
and to study. To develop sensitive
yet rigorous methodologies that
will provide insights into the representational aspect of MRO is an
important future challenge.
MRO and the Family System
The relationship between a parent and child is itself nested in a
network of family relationships.
The importance of studying development in the context of the entire
family system has been increasingly acknowledged. In particular,
future research should study
mother-child and father-child
MRO, both separately and as a triadic interconnected system. More
generally, family-level variables
such as stress, conflict, support,
and affective ambience may be significant dimensions of the context
in which mutually responsive relationships with the child may flourish or fail.
Recommended Reading
Collins, W.A., & Laursen, B. (Eds.).
(1999). (See References)
Kochanska, G. (1997). (See References)
Kochanska, G., & Murray, K.T.
(2000). (See References)
Acknowledgments— This research has
been sponsored by grants from the National Institute of Mental Health (RO1
MH63096, KO2 MH01446) and National
Science Foundation (DBS-9209559, SBR9510863) to the author. I gratefully acknowledge the comments of Nazan Aksan, David Forman, and Robert Siegler,
and contributions of numerous students,
staff, and the families who participated in
the studies.
Note
1. Address correspondence to Grazyna Kochanska, Department of Psychology, University of Iowa, Iowa City,
IA 52242-1447.
References
Clark, M.S. (1984). Record keeping in two types of
relationships. Journal of Personality and Social
Psychology, 47, 549–557.
Collins, W.A., & Laursen, B. (Eds.). (1999). Minnesota Symposia on Child Psychology: Vol. 30. Relationships as developmental contexts. Hillsdale,
NJ: Erlbaum.
Collins, W.A., Maccoby, E.E., Steinberg, L., Hetherington, E.M., & Bornstein, M.H. (2000). Contemporary research on parenting: The case for
nature and nurture. American Psychologist, 55,
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Deater-Deckard, K., & O’Connor, T.G. (2000). Parent-child mutuality in early childhood: Two
behavioral genetic studies. Developmental Psychology, 36, 561–570.
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Eisenberg, N., & Fabes, R.A. (1998). Prosocial development. In W. Damon (Series Ed.) & N.
Eisenberg (Vol. Ed.), Handbook of child psychology: Vol. 3. Social, emotional, and personality development (pp. 701–778). New York: Wiley.
Emde, R.N., Biringen, Z., Clyman, R.B., & Oppenheim, D. (1991). The moral self of infancy: Affective core and procedural knowledge.
Developmental Review, 11, 251–270.
Forman, D.R., & Kochanska, G. (2001). Viewing imitation as child responsiveness: A link between
teaching and discipline domains of socialization. Developmental Psychology, 37, 198–206.
Grusec, J.E. (1997). A history of research on
parenting strategies and children’s internalization of values. In J.E. Grusec & L. Kuczynski
(Eds.), Parenting and children’s internalization of
values: A handbook of contemporary theory (pp.
3–22). New York: Wiley.
Kochanska, G. (1997). Mutually responsive orientation between mothers and their young children: Implications for early socialization. Child
Development, 68, 94–112.
Kochanska, G., Coy, K.C., & Murray, K.T. (2001).
The development of self-regulation in the first
four years of life. Child Development, 72, 1091–
1111.
Kochanska, G., Forman, G., & Coy, K.C. (1999).
Implications of the mother-child relationship
in infancy for socialization in the second year
of life. Infant Behavior and Development, 22, 249–
265.
Kochanska, G., & Murray, K.T. (2000). Motherchild mutually responsive orientation and conscience development: From toddler to early
school age. Child Development, 71, 417–431.
Laible, D.J., & Thompson, R.A. (2000). Motherchild discourse, attachment security, shared
positive affect, and early conscience development. Child Development, 71, 1424–1440.
Maccoby, E.E. (1999). The uniqueness of the parent-child relationship. In W.A. Collins & B.
Laursen (Eds.), Minnesota Symposia on Child
Psychology: Vol. 30. Relationships as developmental contexts (pp. 157–175). Hillsdale, NJ: Erlbaum.
Reis, H.T., Collins, W.A., & Berscheid, E. (2000).
Relationships in human behavior and development. Psychological Bulletin, 126, 844–872.
Copyright © 2002 American Psychological Society
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Current Directions in Psychological
Science
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Ape and Human Cognition : What's the Difference?
Michael Tomasello and Esther Herrmann
Current Directions in Psychological Science 2010 19: 3
DOI: 10.1177/0963721409359300
The online version of this article can be found at:
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Ape and Human Cognition: What’s the
Difference?
Current Directions in Psychological
Science
19(1) 3-8
ª The Author(s) 2010
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DOI: 10.1177/0963721409359300
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Michael Tomasello and Esther Herrmann
Max Planck Institute for Evolutionary Anthropology, Leipzig, Germany
Abstract
Humans share the vast majority of their cognitive skills with other great apes. In addition, however, humans have also evolved a
unique suite of cognitive skills and motivations—collectively referred to as shared intentionality—for living collaboratively,
learning socially, and exchanging information in cultural groups.
Keywords
apes, culture, cognition, evolution, cooperation
Surely one of the deepest and most important questions in all of
the psychological sciences is how human cognition is similar to
and different from that of other primates. The main datum is this:
Humans seemingly engage in all kinds of cognitive activities that
their nearest primate relatives do not, but at the same time there is
great variability among different cultural groups. All groups have
complex technologies but of very different types; all groups use
linguistic and other symbols but in quite different ways; all
groups have complex social institutions but very different ones.
What this suggests is that human cognition is in some way bound
up with human culture. Here we argue that this is indeed the case,
and we then try to explain this fact evolutionarily.
Similarities in Ape and Human Cognition
The five great ape species (orangutans, gorillas, chimpanzees,
bonobos, humans) share a common ancestor from about 15 million years ago, with the last three sharing a common ancestor
from about 6 million years ago (see Fig. 1 for a picture of chimpanzees). Since great apes are so closely related to one another
evolutionarily, it is natural that they share many perceptual,
behavioral, and cognitive skills.
primates’ cognitive skills for dealing with the physical world
almost certainly evolved in the context of foraging for food.
As compared with other mammals, primates may face special
challenges in locating their daily fare, since ripe fruits are patchy resources that are irregularly distributed in space and time.
Other studies suggest that great apes understand their social
worlds in basically the same way as humans as well. Like
humans, apes live in a world of identifiable individuals with
whom they form various kinds of social relationships—for
example, in terms of dominance and ‘‘friendship’’—and they
recognize the third-party social relationships that other individuals have with one another. Moreover, they go beneath this
understanding of social behavior and relationships to understand
the goals and perceptions of social partners acting as intentional
agents (see Call & Tomasello, 2008, for a review). Apes’ and
other primates’ cognitive skills for dealing with the social world
evolved mainly in the context of competition with groupmates
for valued resources, and primates, as compared with other
mammals, live in especially complex social groups (leading to
so-called Machiavellian intelligence; Byrne & Whiten, 1988).
Great ape cognitive operations
Great ape cognitive worlds
Many different studies suggest that nonhuman great apes (hereafter great apes) understand the physical world in basically the
same way as humans. Like humans, apes live most basically in
a world of permanent objects (and categories and quantities of
objects) existing in a mentally represented space. Moreover,
they understand much about various kinds of events in the
world and how these events relate to one another causally (see
Tomasello & Call, 1997, for a review). Apes’ and other
Great apes also operate on their cognitive worlds in ways very
similar to humans. Thus, apes not only perceive and understand
things in the immediate here and now but they also recall things
Corresponding Author:
Michael Tomasello, Max Planck Institute for Evolutionary Anthropology,
Department of Developmental and Comparative Psychology, Deutscher
Platz 6, D-04103 Leipzig, Germany
E-mail: tomas@eva.mpg.de
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4
Tomasello, Herrmann
Fig. 1. Chimpanzees in a social group.
they have perceived in the past and anticipate or imagine things
that might happen in the future. For example, in a recent study,
some great apes used a tool to retrieve food, and when the food
was gone they dropped the tool and left. Later, when they
returned, more food was there but the necessary tool was not.
After only a few repetitions of this procedure, the apes learned
to take the tool with them after using it, in anticipation of the
next trial when they would need it again (Mulcahy & Call,
2006).
Great apes also can make inferences about what one perceived state or event implies about another. For example, in
another experiment, great apes were faced with two cups, and
they knew that only one of them contained food. They then
watched as a human shook one. Not only were they able to infer
which one had food when they heard it in there, they were also
able to infer which one had food (i.e., the other one) when the
shaken cup made no sound. This is a kind of reasoning by
exclusion (analogous to disjunctive syllogism in formal logic):
(a) the food is in one of the cups; (b) it is not in this one
(inferred from lack of sound—causal reasoning); (c) so then
it must be in the other one. The apes thus used their knowledge
and reasoning to imagine the food in the correct cup (Call,
2004).
Apes also can reason about the decision making of other
individuals. For instance, in a recent study, human-raised chimpanzees observed a human successfully solving a problem in a
particular way. The chimpanzees then either followed that way
or not depending on whether their particular circumstances—
that is, the obstacles to solving the problem—were the same
or different as those that had faced the human demonstrator.
They seemingly reasoned about why the human had chosen the
behavioral means she had (Buttelmann, Carpenter, Call, &
Tomasello, 2007).
Differences in Ape and Human Cognition
Since humans have brains three times larger than other great
apes—and share so many basic cognitive skills with them—it
would be natural to assume that humans are just more cognitively sophisticated than apes in a general way. But this is not
the case; the situation is much more interesting than that.
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Ape and Human Cognition
5
An overall comparison
In a recent study, Herrmann, Call, Hernández-Lloreda, Hare, and
Tomasello (2007) gave an extensive battery of cognitive tasks to
large numbers of chimpanzees, orangutans, and 2-year-old
human children. The tasks assessed all kinds of cognitive skills
for dealing with both the physical and the social world. If what
differentiates humans from their nearest primate relatives is
simply a greater degree of general intelligence—better skills
of perceptual discrimination, larger working memories, more
inferencing skills, and so forth—then the children should have
differed from the apes uniformly across all the different kinds
of tasks. But that was not the case. The finding was that the children were very similar to the apes in their cognitive skills for
dealing with space, quantities, and causality; 2-year-olds still
have their same basic great-ape skills for dealing with the physical world. But these same 2-year-old children—still preliterate,
prenumerical, and preschool—showed much more sophisticated
cognitive skills for dealing with the social world in terms of
intention-reading, social learning, and communication.
So early in ontogeny human infants show some quantitative
advantages over apes in social-cognitive skills that they do not
show in other cognitive domains. The proposal is that the children’s special social-cognitive skills represent the dawning of a
special kind of cultural intelligence evolved for participating in
a cultural group. Participating in a cultural group will then
enhance all of children’s cognitive skills across the board,
including those for dealing with the physical world—as children, for example, imitate others’ tool use, acquire a language
and all its conceptual categories, learn mathematical symbols
and operations via instruction, and so forth. Children’s special
skills of social cognition thus bootstrap their skills of physical
cognition by enabling them to collaborate with, communicate
with, and learn from others in the cultural group.
Evolutionarily, the key difference is that humans have
evolved not only social-cognitive skills geared toward competition, but also social-cognitive skills and motivations geared
toward complex forms of cooperation—what we call skills and
motivations for shared intentionality (Tomasello, Carpenter,
Call, Behne, & Moll, 2005). Most important are skills and motivations for shared intentionality in children’s (a) collaboration
and communication and (b) cultural learning and transmission.
The ability to collaborate and communicate with others in
sophisticated, species-unique ways is apparent even in prelinguistic human infants (see Fig. 2). In a recent comparative
study, human 1-year-olds and juvenile chimpanzees each
engaged in a collaborative task with a human adult. When the
adult stopped participating, the chimpanzees simply tried to
solve the task alone. The human children, in contrast, employed
various forms of communication to try to reengage the adult
into the task. The children seemed to understand that the two
of them had committed themselves to doing this together and
it simply would not do if the adult was shirking her duty. The
collaboration was structured by joint goals and joint commitments to one another (Warneken, Chen, & Tomasello, 2006).
It is not difficult to see in these simple activities the roots of the
kind of collaborative commitments and activities that structure
human social institutions, from governments to religions.
And the way humans communicate is fundamentally cooperative as well. Humans do not just try to get others to do what
they want them to—which is what most animal communication
(and much human communication) is about—but they also
communicate simply to inform others of things helpfully and
to share emotions and attitudes with them freely. Human
infants communicate in this cooperative way even before they
acquire language, especially with the pointing gesture
(Tomasello, Carpenter, & Lizskowski, 2007). Human languages, as the pinnacles of human communication, rely on
these cooperative motives as well, but they are also constituted
by fundamentally cooperative communicative devices—
known as linguistic conventions (or symbols)—whose meanings derive from a kind of cooperative agreement that we will
all use them in the same way (Tomasello, 2008).
Both collaborative activities with shared goals and cooperative communication using shared symbols are structured by
joint attention. This means that as children work together with
others or communicate with them, they have a mutual awareness that this is what they are doing: We are both committed
to this joint goal; or, we are both focused on this same object
together. This creates the possibility of culturally constituted
entities that exist because, and only because, everyone in the
group believes and acts as if they do—for example, such things
as marriage and money and presidents (Searle, 1995).
Cultural learning and transmission
Collaboration and communication
Virtually all of humans’ highest cognitive achievements are not
the work of individuals acting alone but rather of individuals
collaborating in groups. Other great apes, especially chimpanzees, coordinate their actions with others in a number of complex ways—for example, in capturing small animals and in
coalitions and alliances in intragroup conflicts (Muller &
Mitani, 2005). But humans collaborate and communicate with
one another in especially complex ways that go beyond simple
coordination, ending up with such things as complex social
institutions structured by joint goals, division of labor, and
communicative symbols.
All great apes, especially chimpanzees and orangutans, transmit some behaviors and information across generations culturally (Whiten & van Schaik, 2007). But the human way of
living depends fundamentally and totally on cultural learning
and transmission. In particular, the human way of living
depends on processes of cultural evolution in which material
and symbolic artifacts and social practices accumulate modifications over time (ratchet up in complexity), such that they
have a ‘‘history’’ within the group upon which others can
always build (Tomasello, 1999).
Much empirical research on social learning and imitation
has shown that young children understand and reproduce, to
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6
Tomasello, Herrmann
Fig. 2. Collaboration in young children.
a greater extent than other apes, not just the environmental
result of others’ actions but also the behavioral and cognitive
processes used to produce that result (Whiten, 2005). In addition, other important aspects of cultural learning in humans
derive from their special cooperative skills and motivations,
and these add to the power of the human cultural ratchet as
well. Specifically, adults teach children things intentionally—
whereas teaching is not an important dimension in the lives
of other great apes, if it exists at all—and teaching is a form
of altruistic cooperation (free donation of information). Human
children are especially attuned to adults teaching them things
(Gergely & Csibra, 2006), and they trust adult instruction
implicitly based on their cooperative motives. Indeed, when
adults teach them things, children trust this so much they
often jump to normative conclusions. Thus, they learn not just
that this is how the adult did it, but that this is how it is done—
this is how we in this group do it, how it ought to be done. For
example, in a recent study, 3-year-old children who witnessed a
puppet playing a game in a manner discrepant with the way
they had been taught objected strenuously: The puppet was not
doing it ‘‘right’’ (Rakoczy, Warneken, & Tomasello, 2008).
Such normative judgments derive, almost certainly, from identifying with the group in terms of how ‘‘we’’ do things.
And so to complement their special skills of collaborating
with others in the moment, human children also come into the
world ready to ‘‘collaborate,’’ as it were, with forebears in their
culture, by adopting their artifacts, symbols, skills, and practices via imitation and instructed learning. Their cooperative
identification with the group leads them to learn not just that
this is a useful way to do things to meet individual goals, but
it is the ‘‘right’’ way to do things, at least for members of this
group. This almost moral dimension makes human cultural
learning especially powerful in comparison to that of their closest primate relatives.
The Coevolution of Human Culture and
Cognition
As compared with their nearest great-ape relatives, humans
occupy an incredibly wide range of environmental niches
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Ape and Human Cognition
7
covering almost the entire planet. To deal with everything
from the Arctic to the tropics, humans as a species have
evolved a highly flexible suite of cognitive skills. But these
are not individual cognitive skills that enable individuals to
survive alone in the tundra or rain forest, but rather they are
social-cognitive skills that enable them to develop, in concert
with others in their cultural groups, creative ways of coping
with whatever challenges may arise. Humans have not only
skills of individual intentional action and cognition but also
skills and motivations for sharing intentions and cognition
with others.
What most clearly distinguishes human cognition from
that of other primates, therefore, is their adaptations for
functioning in cultural groups. Groups of individuals cooperate together to create artifacts and practices that accumulate improvements (rachet up in complexity) over time,
thus creating ever-new cognitive niches (Tomasello, 1999).
Children must be equipped to participate in this process during their development by means of species-unique cognitive
skills for collaboration, communication, and cultural learning. Humans are thus characterized to an inordinate degree
by what has been called niche construction and gene–culture
coevolution (Richerson & Boyd, 2005), as the species has
evolved cognitive skills and motivations enabling them to
function effectively in any one of many different self-built
cultural worlds.
Some important questions for future research include the
following:
!
!
!
!
How precisely do children’s skills of collaboration and imitative learning differ from those of other great apes?
Do great apes teach? Is human teaching part and parcel of
their more cooperative way of communicating, or something different?
What are the differences in motivation and emotion that
contribute to humans’ special cooperative tendencies and
skills?
How do humans’ skills of cultural creation and cultural
learning differ across cultures—especially those that
emerge early in ontogeny?
Recommended Reading
Call, J., & Tomasello, M. (2008). (See References). A concise review
of research on chimpanzee social cognition (theory of mind) over
the past 30 years.
Herrmann, E., Call, J., Hernández-Lloreda, M., Hare, B., & Tomasello, M. (2007). (See References). Results of the administration
of a very large cognitive test battery to large numbers of chimpanzees, orangutans, and 2-year-old human children.
Richerson, P.J., & Boyd, R. (2005). (See References). A comprehensive description of research and theory on the nature of human culture in evolutionary context.
Tomasello, M., Carpenter, M., Call, J., Behne, T., & Moll, H. (2005).
(See References). A theoretical framework for thinking about how
skills and motivations of shared intentionality manifest themselves
during human ontogeny.
Tomasello, M. (2008). (See References). A theoretical account of how
cooperative communication, including conventional languages,
emerged in human evolution.
Declaration of Conflicting Interests
The authors declared no potential conflicts of interest with respect to
the authorship and/or publication of this article.
References
Buttelmann, D., Carpenter, M., Call, J., & Tomasello, M. (2007).
Enculturated chimpanzees imitate rationally. Developmental Science, 10, F31–38.
Byrne, R.W., & Whiten, A. (Eds.). (1988). Machiavellian Intelligence: Social Expertise and the Evolution of Intellect in Monkeys,
Apes, and Humans. New York: Oxford University Press.
Call, J. (2004). Inferences about the location of food in the great apes
(Pan paniscus, Pan troglodytes, Gorilla gorilla, and Pongo pygmaeus). Journal of Comparative Psychology, 118, 232–241.
Call, J., & Tomasello, M. (2008). Do chimpanzees have a
theory of mind: 30 years later. Trends in Cognitive Science,
12, 187–192.
Gergely, G., & Csibra, G. (2006). Sylvia’s recipe: The role of imitation and pedagogy in the transmission of cultural knowledge. In
N.J. Enfield, & S.C. Levinson (Eds.), Roots of human sociality:
Culture, cognition and interaction (pp. 229–255). Oxford, UK:
Berg Press.
Herrmann, E., Call, J., Hernández-Lloreda, M., Hare, B., &
Tomasello, M. (2007). Humans have evolved specialized skills
of social cognition: The cultural intelligence hypothesis. Science,
317, 1360–1366.
Mulcahy, N.J., & Call, J. (2006). Apes save tools for future use. Science, 312, 1038–1040.
Muller, M.N., & Mitani, J.C. (2005). Conflict and cooperation in wild
chimpanzees. Advances in the Study of Behavior, 35, 275–331.
Rakoczy, H., Warneken, F., & Tomasello, M. (2008). The sources of
normativity: Young children’s awareness of the normative structure of games. Developmental Psychology, 44, 875–881.
Richerson, P.J., & Boyd, R. (2005). Not by Genes Alone: How Culture
Transformed Human Evolution. Chicago: University of Chicago
Press.
Searle, J. (1995). The Construction of Social Reality. New York: Free
Press.
Tomasello, M. (1999). The Cultural Origins of Human Cognition.
Cambridge, MA: Harvard University Press.
Tomasello, M. (2008). Origins of Human Communication. Cambridge, MA: MIT Press.
Tomasello, M., & Call, J. (1997). Primate Cognition. New York:
Oxford University Press.
Tomasello, M., Carpenter, M., Call, J., Behne, T., & Moll, H. (2005).
Understanding and sharing intentions: The origins of cultural cognition. Behavioral and Brain Sciences, 28, 675–691.
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Tomasello, Herrmann
Tomasello, M., Carpenter, M., & Lizskowski, U. (2007). A new look
at infant pointing. Child Development, 78, 705–722.
Warneken, F., Chen, F., & Tomasello, M. (2006). Cooperative activities in young children and chimpanzees. Child Development, 77,
640–663.
Whiten, A. (2005). The second inheritance system of chimpanzees and
humans. Nature, 437, 52–55.
Whiten, A., & van Schaik, C.P. (2007). The evolution of animal
‘cultures’ and social intelligence. Philosophical Transactions of
the Royal Society B, 362, 603–620.
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Current Directions in Psychological
Science
http://cdp.sagepub.com/
Three Laws of Behavior Genetics and What They Mean
Eric Turkheimer
Current Directions in Psychological Science 2000 9: 160
DOI: 10.1111/1467-8721.00084
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160
VOLUME 9, NUMBER 5, OCTOBER 2000
Three Laws of Behavior Genetics and
What They Mean
Eric Turkheimer1
to the implications of the genetics
of behavior for an understanding
of complex human behavior and its
development.
Department of Psychology, University of Virginia, Charlottesville, Virginia
Abstract
Behavior genetics has demonstrated that genetic variance
is an important component of
variation for all behavioral outcomes, but variation among
families is not. These results
have led some critics of behavior genetics to conclude that
heritability is so ubiquitous as
to have few consequences for
scientific understanding of development, while some behavior genetic partisans have
concluded that family environment is not an important cause
of developmental outcomes.
Both views are incorrect. Genotype is in fact a more systematic source of variability than
environment, but for reasons
that are methodological rather
than substantive. Development
is fundamentally nonlinear,
interactive, and difficult to control experimentally. Twin studies offer a useful methodological shortcut, but do not show
that genes are more fundamental than environments.
Keywords
genes; environment; development; behavior genetics
The nature-nurture debate is
over. The bottom line is that everything is heritable, an outcome that
has taken all sides of the naturenurture debate by surprise. Irving
Gottesman and I have suggested
that the universal influence of
genes on behavior be enshrined as
the first law of behavior genetics
(Turkheimer & Gottesman, 1991),
and at the risk of naming laws that
I can take no credit for discovering,
it is worth stating the nearly unanimous results of behavior genetics
in a more formal manner.
● First Law. All human behavioral
traits are heritable.
● Second Law. The effect of being
raised in the same family is
smaller than the effect of genes.
● Third Law. A substantial portion
of the variation in complex human behavioral traits is not accounted for by the effects of
genes or families.
It is not my purpose in this brief
article to defend these three laws
against the many exceptions that
might be claimed. The point is that
now that the empirical facts are in
and no longer a matter of serious
controversy, it is time to turn attention to what the three laws mean,
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VARIANCE AND
CAUSATION IN
BEHAVIORAL
DEVELOPMENT
If the first two laws are taken literally, they seem to herald a great
victory for the nature side of the
old debate: Genes matter, families
do not. To understand why such
views are at best an oversimplification of a complex reality, it is necessary to consider the newest wave
of opposition that behavior genetics has generated. These new critics, whose most articulate spokesman is Gilbert Gottlieb (1991, 1992,
1995), claim that the goal of developmental psychology is to specify
the actual developmental processes
that lead to complex outcomes. In
lower animals, whose breeding
and environment can be brought
under the control of the scientist, it
is possible to document such developmental processes in exquisite detail. The critics draw an unfavorable comparison between these
detailed animal studies and twin
studies of behavior genetics, which
produce only statistical conclusions about the relative importance
of genes and environment in development.
The greatest virtue of the new
challenge is that it abandons the
CURRENT DIRECTIONS IN PSYCHOLOGICAL SCIENCE
implausible environmentalist contention that important aspects of
behavior will be without genetic
influence. Gottlieb (1992) stated,
“The present . . . viewpoint holds
that genes are an inextricable component of any developmental system, and thus genes are involved in
all traits” (p. 147). Unlike earlier
critics who deplored the reductionism they attributed to behavior
genetic theories of behavior, the
developmental biologists take behavior genetics to task for not being mechanistic enough. Once vilified as the paragon of determinist
accounts of human behavior, behavior genetics is now chastised for
offering vague and inconclusive
models of development (Gottlieb,
1995; Turkheimer, Goldsmith, &
Gottesman, 1995), and judged by
the standards of developmental
psychobiology in lower animals, it
is true enough that behavior genetic theories of complex human
behavior seem woefully poorly
specified. But ultimately the charge
is unfair, because there is no
equivalent in developmental psychobiology to the behavior genetic
study of marital status or school
performance. The great preponderance of the exquisite experimental
science that goes into animal psychobiology is quite simply impossible to conduct in humans.
Human developmental social
science is difficult—equally so for
the genetically and environmentally inclined—because of the
(methodologically vexing, humanistically pleasing) confluence of
two conditions: (a) Behavior
emerges out of complex, nonlinear
developmental processes, and (b)
ethical considerations prevent us
from bringing most human de-
161
velopmental processes under effective experimental control. Figure 1
is a schematic illustration of the
problem. Individual genes (Genes
1, 2, and 3) and their environments
(which include other genes) interact to initiate a complex developmental process that determines
adult personality. Most characteristic of this process is its interactivity:
Subsequent environments to which
the organism is exposed depend on
its earlier states, and each new environment changes the developmental trajectory, which affects future expression of genes, and so
forth. Everything is interactive, in
the sense that no arrows proceed
uninterrupted from cause to effect;
any individual gene or environmental event produces an effect
only by interacting with other
genes and environments.
For the behavior geneticist,
Fig. 1. Schematic diagram of contrasting roles of genes and environment in development of personality. One-headed arrows link
causes to effects; two-headed arrows indicate correlations. Genes and environments are both causal inputs into an interactive
developmental system (represented by the network of arrows in the center of the figure), but because people select and shape their
own environments (as represented by lighter one-headed arrows from personality to environments), correlations across the
developmental system (dotted two-headed arrows) are easier to detect for genes than for environments.
Copyright © 2000 American Psychological Society
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162
however, the quasi-experimental
gift of genetically identical and
nonidentical twins offers a remarkable, if deceptively simple, method
to span this daunting interactive
complexity. Thanks to the fact that
identical twins are on average exactly twice as similar genetically as
nonidentical twins, one can use
straightforward statistical procedures to estimate the proportion of
variability in complex outcomes
that is associated with causally distant genes, all the while maintaining a state of near-perfect ignorance about the actual causal
processes that connect genes to behavior. This methodological shortcut is not available to rivals of behavior genetics who seek to
measure the effects of families on
behavior. How similar was my
rearing environment to that of my
siblings? And how similar was it to
the environment of my adopted
sibling, if I have one, or to the environment of my biological sibling
who was raised by someone else?
The apparent victory of nature
over nurture suggested by the first
two laws is thus seen to be more
methodological than substantive.
In a world in which there were occasional occurrences of “identical
environmental twins,” whose experiences were exactly the same,
moment by moment, and another
variety who shared exactly (but
randomly) 50% of their experiences, environmentalists could reproduce the precision of their rivals, and like the behavior
geneticists could measure with
great precision the total contribution of the environment while
knowing almost nothing about the
developmental processes that underlie it.
The old-fashioned nature-nurture debate was about whether or
not genes influence complex behavioral outcomes, and that question has been decisively answered
in the affirmative. The new question is how we can proceed from
VOLUME 9, NUMBER 5, OCTOBER 2000
partitioning sources of variance to
specifying concrete developmental
processes (Turkheimer, 1998), and
although critics like Gottlieb are
correct that heritability per se has
few implications for a scientific understanding of development, they
have failed to emphasize two crucial points. First, heritability does
have one certain consequence: It is
no longer possible to interpret correlations among biologically related family members as prima facie evidence of sociocultural causal
mechanisms. If the children of depressed mothers grow up to be depressed themselves, it does not
necessarily demonstrate that being
raised by a depressed mother is itself depressing. The children might
have grown up equally depressed
if they had been adopted and
raised by different mothers, under
the influence of their biological
mother’s genes. For every behavior
geneticist who continues to report
moderate heritabilities as though
they were news, there is an environmentalist who reports causally
ambiguous correlations between
genetically related parents and
children. Second, the problem the
critics have uncovered extends
well beyond behavior genetics: It is
a rare environmentalist who has
never used statistical methods to
predict behavioral outcomes from
earlier events, in the hope that the
specific developmental mechanisms can be filled in later. The disconnect between the analysis of
variance and the analysis of causes,
to use Lewontin’s (1974) phrase, is
not a proprietary flaw in behavior
genetic methodology; in fact, it is
the bedrock methodological problem of contemporary social science.
NONSHARED
ENVIRONMENT AND THE
GLOOMY PROSPECT
Even after the effects of genes
and the shared effects of families
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have been accounted for, around
50% of the differences among siblings is left unexplained. In recent
years, scientists interested in the
genetics of behavior have come to
call this unexplained portion the
“nonshared environment.” Although according to the second
law shared environment accounts
for a small proportion of the variability in behavioral outcomes, according to the third law, nonshared
environment usually accounts for a
substantial portion. So perhaps the
appropriate conclusion is not so
much that the family environment
does not matter for development,
but rather that the part of the family environment that is shared by
siblings does not matter. What
does matter is the individual environments of children, their peers,
and the aspects of their parenting
that they do not share. Plomin and
Daniels (1987) reviewed evidence
of the predominance of nonshared
environmental variance and posed
a seminal question: Why are children in the same family so different? They proposed that siblings
are different because nonshared
environmental events are more
potent causes of developmental
outcomes than the shared environmental variables, like socioeconomic status, that have formed the
traditional basis of sociocultural
developmental psychology.
Plomin and Daniels’s explanation involves a subtle conceptual
shift, best described in terms of a
distinction between the objective
and effective environment (Goldsmith, 1993; Turkheimer & Waldron, 2000). What qualifies an environmental event as nonshared?
There are two possibilities. The
first is objective: An event is nonshared if it is experienced by only
one sibling in a family, regardless
of the consequences it produces.
The other possibility is effective:
An environmental event is nonshared if it makes siblings different
CURRENT DIRECTIONS IN PSYCHOLOGICAL SCIENCE
rather than similar, regardless of
whether it was experienced by one
or both of them. Plomin and
Daniels’s proposal, then, is that the
nonshared environment as an effectively defined variance component can be explained by objectively nonshared environmental
events. The question, “Why are
children in the same family so different?” is answered, “Because
measurable differences in their environments make them that way.”
This proposal has been enormously influential, spawning an
entire area of empirical inquiry into
the consequences of measured environmental differences among
siblings. Ironically, that same literature has quite decisively demonstrated that the conjecture is
false. A review of 43 studies that
measured differences in the environments of siblings and related
them to differences in the siblings’
developmental outcomes (Turkheimer & Waldron, 2000) has
shown that although upwards of
50% of the variance in behavioral
outcomes is accounted for by the
effectively defined variance component called nonshared environment, the median percentage accounted for by objectively defined
nonshared events is less than 2%.
What could be going on?
Plomin and Daniels (1987) almost identified the answer to this
question, but dismissed it as too
pessimistic:
One gloomy prospect is that the salient
environment might be unsystematic,
idiosyncratic, or serendipitous events
such as accidents, illnesses, or other
traumas . . . . Such capricious events,
however, are likely to prove a dead end
for research. More interesting heuristically are possible systematic sources of
differences between families. (p. 8)
The gloomy prospect is true. Nonshared environmental variability
predominates not because of the
systematic effects of environmental
events that are not shared among
siblings, but rather because of the
unsystematic effects of all environmental events, compounded by the
equally unsystematic processes
that expose us to environmental
events in the first place (Turkheimer & Gottesman, 1996).
A model of nonshared variability based on the gloomy prospect is
radically different from the Plomin
model based on systematic consequences of environmental differences among siblings. Most important, the two models suggest very
different prospects for a genetically
informed developmental psychology. Again and again, Plomin and
his colleagues have emphasized
that the importance of nonshared
environment implies that it is time
to abandon shared environmental
variables as possible explanations
of developmental outcomes. And
although modern environmentalists might not miss coarse measures like socioeconomic status, it
is quite another thing to give up on
the causal efficaciousness of normal families, as Scarr (1992), Rowe
(1994), and Harris (1998) have
urged. If, however, nonshared environmental variability in outcome
is the result of the unsystematic
consequences of both shared and
nonshared environmental events,
the field faces formidable methodological problems—Plomin and
Daniels’s gloomy prospect—but
need not conclude that aspects of
families children share with siblings are of no causal importance.
CONCLUSION:
ANTICIPATING THE
GENOME PROJECT
It is now possible for behavior
genetics to move beyond statistical
analyses of differences between
identical and nonidentical twins
and identify individual genes that
are related to behavioral outcomes.
Copyright © 2000 American Psychological Society
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163
What should we expect from this
endeavor? Behavior geneticists anticipate vindication: At long last,
statistical variance components
will be rooted in the actual causal
consequences of actual genes. Critics of behavior genetics expect the
opposite, pointing to the repeated
failures to replicate associations between genes and behavior as evidence of the shaky theoretical underpinnings of which they have so
long complained.
There is an interesting parallel
between the search for individual
genes that influence behavior and
the failed attempt to specify the
nonshared environment in terms of
measured environmental variables.
In each case, investigators began
with statistically reliable but causally vague sources of variance, and
set out to discover the actual causal
processes that produced them. The
quest for the nonshared environment, as we have seen, got stuck in
the gloomy prospect. Although individual environmental events influence outcomes in the most general sense, they do not do so in a
systematic way. One can detect
their effects only by accumulating
them statistically, using twins or
adoptees.
If the underlying causal structure of human development is
highly complex, as illustrated in
Figure 1, the relatively simple statistical procedures employed by
developmental psychologists, geneticists, and environmentalists
alike are being badly misapplied.
But misapplied statistical procedures still produce what appear to
be results. Small relations would
still be found between predictors
and outcomes, but the underlying
complex causal processes would
cause the apparent results to be
small, and to change unpredictably
from one experiment to the next. So
individual investigators would obtain “results,” which would then
fail to replicate and accumulate
into a coherent theory because the
164
simple statistical model did not fit
the complex developmental process to which it was being applied.
Much social science conducted in
the shadow of the gloomy prospect
has exactly this flavor (e.g., Meehl,
1978).
The gloomy prospect looms
larger for the genome project than
is generally acknowledged. The
question is not whether there are
correlations to be found between
individual genes and complex behavior—of course there are—but
instead whether there are domains
of genetic causation in which the
gloomy prospect does not prevail,
allowing the little bits of correlational evidence to cohere into replicable and cumulative genetic
models of development. My own
prediction is that such domains
will prove rare indeed, and that the
likelihood of discovering them will
be inversely related to the complexity of the behavior under
study.
Finally, it must be remembered
that the gloomy prospect is gloomy
only from the point of view of the
working social scientist. Although
frustrated developmental psychologists may be tempted to favor
methodologically tractable heuris-
VOLUME 9, NUMBER 5, OCTOBER 2000
tics over chaotic psychological reality, it is a devil’s choice: In the
long run, the gloomy prospect always wins, and no one would want
to live in a world where it did not.
Psychology is at least one good
paradigm shift away from an empirical answer to the gloomy prospect, but the philosophical response is becoming clear: The
additive effect of genes may constitute what is predictable about
human development, but what is
predictable about human development is also what is least interesting about it. The gloomy prospect
isn’t.
Recommended Reading
Gottlieb, G. (1992). (See References)
Lewontin, R.C. (1974). (See References)
Meehl, P.E. (1978). (See References)
Plomin, R., & Daniels, D. (1987). (See
References)
Note
1. Address correspondence to Eric
Turkheimer, Department of Psychology, 102 Gilmer Hall, P.O. Box 400400,
University of Virginia, Charlottesville,
VA 22904-4400; e-mail: turkheimer@
virginia.edu.
Published by Blackwell Publishers Inc.
Downloaded from cdp.sagepub.com at OhioLink on January 7, 2013
References
Goldsmith, H. (1993). Nature-nurture issues in the
behavioral genetic context: Overcoming barriers to communication. In R. Plomin & G. McClearn (Eds.), Nature, nurture and psychology
(pp. 325–339). Washington, DC: American
Psychological Association.
Gottlieb, G. (1991). Experiential canalization of behavioral development: Theory. Developmental
Psychology, 27, 4–13.
Gottlieb, G. (1992). Individual development and evolution. New York: Oxford University Press.
Gottlieb, G. (1995). Some conceptual deficiencies
in “developmental” behavior genetics. Human
Development, 38, 131–141.
Harris, J.R. (1998). The nurture assumption: Why
children turn out the way they do. New York:
Free Press.
Lewontin, R.C. (1974). The analysis of variance
and the analysis of causes. American Journal of
Human Genetics, 26, 400–411.
Meehl, P.E. (1978). Theoretical risks and tabular
asterisks: Sir Karl, Sir Ronald, and the slow
progress of soft psychology. Journal of Consulting and Clinical Psychology, 46, 806–834.
Plomin, R., & Daniels, D. (1987). Why are children
in the same family so different from one another? Behavioral and Brain Sciences, 10, 1–60.
Rowe, D.C. (1994). The limits of family influence:
Genes, experience, and behavior. New York: Guilford Press.
Scarr, S. (1992). Developmental theories for the
1990s: Development and individual differences. Child Development, 63, 1–19.
Turkheimer, E. (1998). Heritability and biological
explanation. Psychological Review, 105, 782–791.
Turkheimer, E., Goldsmith, H.H., & Gottesman,
I.I. (1995). Commentary. Human Development,
38, 142–153.
Turkheimer, E., & Gottesman, I.I. (1991). Is H2 = 0
a null hypothesis anymore? Behavioral and
Brain Sciences, 14, 410–411.
Turkheimer, E., & Gottesman, I.I. (1996). Simulating the dynamics of genes and environment in
development. Development and Psychopathology, 8, 667–677.
Turkheimer, E., & Waldron, M.C. (2000). Nonshared environment: A theoretical, methodological, and quantitative review. Psychological
Bulletin, 126, 78–108.
Current Directions in Psychological
Science
http://cdp.sagepub.com/
Genetic and Environmental Influences on Cognition Across Development and Context
Elliot M. Tucker-Drob, Daniel A. Briley and K. Paige Harden
Current Directions in Psychological Science 2013 22: 349
DOI: 10.1177/0963721413485087
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research-article2013
CDPXXX10.1177/0963721413485087Tucker-Drob et al.Genetics of Cognition
Genetic and Environmental Influences on
Cognition Across Development and Context
Current Directions in Psychological
Science
22(5) 349–355
© The Author(s) 2013
Reprints and permissions:
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DOI: 10.1177/0963721413485087
cdps.sagepub.com
Elliot M. Tucker-Drob, Daniel A. Briley, and K. Paige Harden
Department of Psychology and Population Research Center, University of Texas at Austin
Abstract
Genes account for between approximately 50% and 70% of the variation in cognition at the population level.
However, population-level estimates of heritability potentially mask marked subgroup differences. We review the
body of empirical evidence indicating that (a) genetic influences on cognition increase from infancy to adulthood,
and (b) genetic influences on cognition are maximized in more advantaged socioeconomic contexts (i.e., a Gene ×
Socioeconomic Status interaction). We discuss potential mechanisms underlying these effects, particularly transactional
models of cognitive development. Transactional models predict that people in high-opportunity contexts actively
evoke and select positive learning experiences on the basis of their genetic predispositions; these learning experiences,
in turn, reciprocally influence cognition. The net result of this transactional process is increasing genetic influence with
increasing age and increasing environmental opportunity.
Keywords
cognitive ability, intelligence, Gene × Environment interaction, behavior genetics, cognitive development
Intelligence is mostly a matter of heredity, as we
know from studies of identical twins reared apart.
. . . Social programs that seek to raise I.Q. are bound
to be futile. Cognitive inequalities, being written in
the genes, are here to stay, and so are the social
inequalities that arise from them. What I have just
summarized, with only a hint of caricature, is the
hereditarian view of intelligence.
—Jim Holt, New York Times Sunday
Book Review, March 27, 2009
In modern industrialized populations, cognition is
approximately 50% to 70% heritable (Bouchard & McGue,
1981). This means that genetic differences between people account for 50% to 70% of the variation in performance on tests of cognitive abilities, such as reasoning,
memory, processing speed, mental rotation, and knowledge. These heritability estimates are based on studies of
identical and fraternal twins raised together, identical
twins separated at birth and raised apart, and adoptive
families. All of these designs hinge on the question of
whether more genetically related individuals (e.g., biological siblings versus adoptive siblings) are also more
similar in their cognitive ability. More recently, molecular
genetic studies of unrelated persons have converged on
similar heritability estimates (Chabris et al., 2012; Davies
et al., 2011). Despite the vociferous objections of critics
of behavioral genetic research (e.g. Charney, 2012),
whether genetic differences between individuals account
for variation in cognition is no longer a question of serious scientific debate. As McGue (1997, p. 417) commented, “That the debate now centres on whether IQ is
50% or 70% heritable is a remarkable indication of how
the nature-nurture question has shifted.”
These heritability estimates have been interpreted—
both by scientists and by the lay public—to mean that
environmental experiences have a minimal impact on
cognition. In this article, we describe an alternative interpretation of what it means for cognition to be heritable:
Rather than rendering environments impotent, genetic
influences on cognition are the result of accumulating
environmental experiences and depend on exposure to
high-quality environmental contexts over time.
Corresponding Author:
Elliot M. Tucker-Drob, Department of Psychology, University of Texas
at Austin, 108 E. Dean Keeton Stop A8000, Austin, TX 78712-1043
E-mail: tuckerdrob@utexas.edu
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Tucker-Drob et al.
350
An “Educational” Example: The
Heritability of Educational Attainment
in the 20th Century
To illustrate how genetic influences on psychosocial outcomes can depend on the environment, we begin with
an example involving generational differences in educational attainment. After World War II, there was a dramatic expansion of access to education in Norway. In
1960, the average educational attainment for Norwegian
adults was 5.92 years; by 2000, it was 11.86 years (Barro
& Lee, 2000). This expansion was driven by postwar
increases in government-sponsored student loans and by
a social climate that increasingly valued education
(Kuhnle, 1986). In contrast, prewar educational opportunities in Norway were less universal, and educational
attainment was much more dependent on family social
class. Over this same period, the heritability of educational attainment nearly doubled, from 40% for Norwegian
male twins born before 1940 to approximately 70% for
those born after 1940 (Heath et al., 1985).
If it were indeed the case, as suggested by the New
York Times quote above, that heritability imposes an
upper limit on the effectiveness of social change, then
why would sweeping social changes be accompanied by
an increase in both the level and the heritability of educational attainment? One explanation is that, as social
opportunity increases, a person’s educational attainment
becomes increasingly a function of his or her individual
characteristics—interests, motivation, work ethic, and
scholastic aptitude—rather than social position. To the
extent that these individual characteristics reflect genetic
differences between people, however slight, then the net
result of individuals’ selecting their own educational
paths is greater heritability of educational attainment.
This explanation implies that heritability is maximized
when people are free to select their own experiences.
This same process may be a key mechanism for cognitive
development.
Transactional Models of Cognitive
Development
Gene-environment correlation—in which environmental
experiences become sorted on the basis of individuals’
genetically influenced traits—is not specific to educational attainment. Rather, behavioral genetic studies have
found that a broad array of presumably “environmental”
experiences—such as negative life events, relationships
with parents, and experiences with peers—are themselves heritable (Kendler & Baker, 2007). That is, genetically similar people (such as monozygotic twins)
experience more similar environments, whereas genetically dissimilar people (such as adoptive siblings) experience less similar environments.
Transactional models posit that these gene-environment correlations are key mechanisms of cognitive development. Early genetically influenced behaviors lead a
person to select (and to be selected into) particular types
of environments; these environments, in turn, have causal
effects on cognition and serve to reinforce the original
behaviors that led to those experiences. As Dickens and
Flynn (2001, p. 347) stated, “higher IQ leads one into better environments causing still higher IQ, and so on.” In
addition to early cognitive ability, “noncognitive” traits,
such as motivation and intellectual interest, may also lead
children into cognition-enhancing environments (TuckerDrob & Harden, 2012b). For instance, higher achievement motivation may lead students to enroll in more
challenging courses, spend free time engrossed in intellectually stimulating activities, and engage parents, peers,
and teachers in more sophisticated discourse.
Longitudinal research has documented bidirectional
associations consistent with transactional processes. For
example, not only does greater parental stimulation predict children’s subsequent test scores, but children’s test
scores also predict higher subsequent stimulation by
parents (e.g., Lugo-Gil & Tamis-LeMonda, 2008; TuckerDrob & Harden, 2012a). Moreover, children’s dispositions
toward engaging with stimulating learning environments
predict later test scores, and children’s test scores predict
their later dispositions toward learning (Marsh, Trautwein,
Lüdtke, Köller, & Baumert, 2005). Such positive feedback
loops may yield increasing dividends. If genes influence
a child’s early behaviors, even small initial genetic differences can be compounded via gene-environment
correlation, leading to large estimates of genetic effects.
In this way, the genetic effects on individual differences
in psychological development can depend on reciprocal transactions with the environment. As Scarr and
McCartney (1983) explained,
We do not think that development is precoded in
the genes and merely emerges with maturation.
Rather, we stress the role of the genotype in determining which environments are actually experienced and what effects they have on the developing
person. (p. 425)
Transactional models propose that genetic differences
between people matter for cognition because initial
genetic differences lead to different environmental experiences. The “end state” of this transactional process—
high levels of and high heritability of cognitive ability—is
therefore expected to differ depending on the quality
and availability of environmental experiences. Thus, differences in heritability between groups can provide
important information about the developmental processes undergirding cognition. Contemporary research in
behavioral genetics of cognition has identified two
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Genetics of Cognition
351
dimensions along which heritability differs: age/
development and socioeconomic advantage. Below, we
summarize results from these two streams of research
and describe how these results can be understood within
the framework of transactional models.
Developmental Changes in Heritability
Children are born with all of their genes, and they experience an ever-wider array of environmental inputs as they
develop. One might therefore expect that genetic variation
will account for less and less variation in psychological
outcomes with age. However, in contrast to this intuitive
hypothesis, genetic influences on cognition actually
increase substantially with age. Aggregated results from 11
unique longitudinal twin and adoption studies of cognition are shown in Figure 1. In infancy, genes account for
less than 25% of the variation in cognition, whereas the
shared family environment accounts for approximately
60%. By adolescence, however, genes account for approximately 70% of the variation in cognition, and the shared
environment accounts for virtually no variation. These
age-related patterns were identified in cross-sectional analyses originally by McCartney, Harris, and Bernieri (1990)
and McGue, Bouchard, Iacono, and Lykken (1993), and
more recently by Haworth et al. (2009).
We can understand the developmental increase in the
heritability of cognition within the transactional framework. As children select and evoke experiences in line
with their genetic predispositions, and as these experiences, in turn, stimulate their cognitive development,
early genetic influences on cognition will become amplified. This compounding process is expected to become
accelerated as children gain increasingly more autonomy
in selecting their peer groups, afterschool activities, academic courses, and other positive learning experiences.
A second explanation for the developmental increase in
heritability is that “new” genes that did not previously
influence cognition may become activated later in development. For example, the biological changes of puberty
may trigger changes in gene expression, or genetic differences that were not previously relevant for cognition may
become relevant as children’s social contexts change. In
fact, both “new” gene activation and gene-environment
transactions may contribute to developmental increases in
the heritability of cognition, and the relative importance of
each process may differ across the lifespan. Longitudinal
behavioral genetic studies have indicated that activation of
“new” genes may be the primary mechanism underlying
increasing heritability in early childhood, whereas transactional processes may be the primary mechanism underlying increasing heritability in middle childhood and
adolescence (Briley & Tucker-Drob, in press).
Unfortunately, much of what is known about the
behavioral genetics of cognitive development has been
Fig. 1. Proportion of variance in cognition as a function of age. Shading around each line represents the imprecision of the estimate (± 1 SE).
The family environment, often termed the shared environment, represents environmental influences that make siblings raised in the same
family more similar to one another. The unique environment, often
termed the nonshared environment, represents environmental influences that differentiate siblings raised in the same family. Data were
aggregated from published reports, based on 11 unique longitudinal
twin and adoption samples (weighted by the precision of the individual
estimates): the Colorado Adoption Project (Petrill et al., 2004), the Early
Childhood Longitudinal Study—Birth Cohort (Tucker-Drob, Rhemtulla,
Harden, Turkheimer, & Fask, 2011), the Longitudinal Twin Study (Bishop
et al., 2003), the Louisville Twin Project (McArdle, 1986), the MacArthur
Longitudinal Twin Study (Cherny et al., 2001), a Moscow community
sample (Malykh, Zyrianova, & Kuravsky, 2003), the Netherlands Twin
Registry (Hoekstra, Bartels, & Boomsma, 2007; Polderman et al., 2006;
van Soelen et al., 2011), the Twins Early Development Study (Davis,
Haworth, & Plomin, 2009), and the Western Reserve Reading Project
(Hart, Petrill, Deater-Deckard, & Thompson, 2007). Articles were identified by searching abstracts in PsycINFO. From the search results, we
included longitudinal studies with samples of siblings with varying
degrees of genetic relatedness, complete cross-time and within-time
sibling correlations (or parameters from behavioral genetic models
producing expectations for these correlations), measurement using an
objective cognition/intelligence test, and participants under age 19 at
both baseline and at least one follow-up measurement occasion.
derived from convenience samples of twins in the United
States and from representative samples of twins from less
racially and socioeconomically diverse populations.
Thus, the trend of increasing heritability with age may
not apply as well to groups with low socioeconomic
status (SES). Next we discuss emerging research on the
question of whether the heritability of cognition differs as
a function of SES.
Socioeconomic Differences in
Heritability
Under a transactional model of cognitive development,
children are expected to select and evoke their environmental experiences on the basis of genetically influenced
dispositions, but this process depends on the existence
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Tucker-Drob et al.
352
of adequate opportunities for such experiences. SES,
which is typically measured using parental income, educational attainment, occupational status, or some combination of the three, is an omnibus marker of the quality
of environmental opportunity. In high-SES contexts, children have abundant opportunities to select and evoke
positive learning experiences on the basis of their genetically influenced motivations and proclivities. In low-SES
contexts, children are less likely to receive adequate
opportunities for cognitively stimulating experiences,
both at home and in school. For example, children from
disadvantaged backgrounds typically have less access to
enriching books and other learning materials, less rigorous academic experiences, and lower quality interactions
with both peers and adults (Duncan & Murnane, 2011).
Because low-SES contexts do not support transactional
processes, it is predicted that genetic potentials for cognitive development are not fully realized (Bronfenbrenner
& Ceci, 1994).
Indeed, research on Gene × SES interaction has indicated that genetic influences on cognition are suppressed
by socioeconomic disadvantage. For children in low-SES
contexts, the heritability of cognition approaches zero,
whereas for children in advantaged contexts, genes
account for as much as 80% of individual differences in
cognition (see Fig. 2). This Gene × SES interaction has
been found in young children (Scarr-Salapatek, 1971;
Turkheimer, Haley, Waldron, D’Onofrio, & Gottesman,
2003), adolescents (Harden, Turkheimer, & Loehlin, 2007;
Rowe, Jacobson, & van den Oord, 1999), and adults
(Bates, Lewis, & Weiss, in press). Moreover, although
socioeconomic disparities in cognition and achievement
are often interpreted as being the result of inequalities in
education, Tucker-Drob, Rhemtulla, Harden, Turkheimer,
and Fask (2011) found evidence for a Gene × SES interaction on infants’ cognitive development between 10
months and 2 years of age, more than 3 years before the
typical age of kindergarten entry. Specifically, for children in high-SES homes, genetic influences on cognition
increased from approximately 0% at 10 months to 50% at
2 years, whereas for children in low-SES homes, genetic
influences on infant cognition remained very close to 0%
across the study period. That is, disadvantaged children
did not show the expected developmental increase in the
heritability of cognition. In follow-up work with this sample, a similar Gene × SES interaction was found on
school-readiness skills (specifically mathematics) at age
4 years (Rhemtulla & Tucker-Drob, 2012). However, the
interaction at 4 years was found to be entirely independent of the Gene × SES interaction earlier in development. This result suggests that Gene × SES interactions
on cognition occur throughout infancy and early childhood, not because early life disadvantages have left
indelible effects on cognition, but rather because low SES
Fig. 2. Variance in mental ability as a function of SES in late infancy (age 2 years) (A). Data come from a nationally representative sample of American twins, 25% of whom lived below the poverty line (Tucker-Drob, Rhemtulla, Harden, Turkheimer, & Fask, 2011). Variance in cognitive aptitude
as a function of parental income in adolescence (age 17 years) (B). Data come from a positively selected sample of adolescent twins who sat for the
National Merit Scholarship Qualifying Test (Harden, Turkheimer, & Loehlin, 2007), very few of whom were likely to be living in poverty. Because a
Gene × SES interaction was detected in this more positively selected sample, Harden et al. (2007) concluded that “genotype-by-environment interactions in cognitive development are not limited to severely disadvantaged environments, as has been previously suggested.” Shading around each
line represents the imprecision of the estimate (± 1 SE). The family environment, often termed the shared environment, represents environmental
influences that make siblings raised in the same family more similar to one another. The unique environment, often termed the nonshared environment, represents environmental influences that differentiate siblings raised in the same family.
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Genetics of Cognition
353
children are recurrently exposed to poor environments
that have novel, yet analogous, interactions with their
genes at different ages.
Although a number of studies have replicated Gene ×
SES interactions on cognition, a handful of notable studies with sound designs have failed to replicate these
effects (see Hanscombe et al., 2012 for a review). It is
noteworthy that these failures to replicate have predominantly been in northern European nations, where social
welfare systems are more comprehensive, whereas most
of the positive results have been obtained in the United
States, where social class differences in educational
opportunity are vast. Socioeconomic disadvantage may
not disrupt gene-environment transactions to the same
extent in countries that ensure access to adequate medical care and high-quality education. Future research
should identify the specific circumstances in which these
Gene × SES interactions hold, by taking into account
both macroenvironmental contexts (e.g., regional and
national characteristics) and school- and family-level differences in economic opportunity and constraint.
Conclusions and Outlook
The results reviewed here suggest a provocative reconceptualization of the relationship between social opportunity
and the magnitude of heritable variation in cognition. We
began this article with a quote that illustrates the common
view that heritability estimates provide an “upper bound”
on the effects of social intervention—if cognition is very
heritable, then the environment cannot matter as much. In
fact, research on how the heritability of cognition differs
across development and across context suggests that
genetic influences on cognition are maximized by environmental opportunity. The highest heritability estimates
are obtained for older children and adolescents from economically advantaged homes—that is, among children
who have the autonomy to select environmental experiences consistent with their own interests and who have an
array of high-quality experiences to choose from. As social,
educational, and economic opportunities increase in a
society, genetic differences will account for increasing
variation in cognition—and perhaps ultimately in educational and economic attainment.
Distinguishing transactional processes from the “direct”
influences of genes is more than a simple academic exercise. As Plomin, DeFries, and Loehlin (1977) wrote:
Although formally it may not matter one whit in
which way the effects of the genes are mediated, in
practice it often matters quite a few whits, especially
if one should happen to be interested in intervening
in the process. (p. 321)
Indeed, child-driven transactions may be critical
for intervention success. For example, Epps and Huston
(2007) found that a poverty intervention changed parenting behaviors indirectly through effects on child
behaviors; there was no immediate, direct effect of
the intervention on parenting behaviors. In other words,
the intervention was unable to directly influence parents to provide higher quality care but was able
to change child behaviors to evoke more effective
care from their parents. By determining the specific
environmental transactions that amplify genetic influences across development and across contexts, researchers may uncover new opportunities for environmental
intervention.
Recommended Reading
Bronfenbrenner, U., & Ceci, S. J. (1994). (See References).
A theoretical account of how social context and genetic
potentials interact to influence positive developmental outcomes.
Dickens, W. T., & Flynn, J. R. (2001). (See References). An indepth treatment of how transactional processes can cause
small genetic differences to be amplified to result in large
estimates of heritability.
Nisbett, R. E., Aronson, J., Blair, C., Dickens, W., Flynn, J.,
Halpern, D. F., & Turkheimer, E. (2012). Intelligence:
New findings and theoretical developments. American
Psychologist, 67, 130–159. A survey of the past 15 years of
research on intelligence, including a section containing a
review of empirical studies on “Social Class and Heritability
of Cognitive Ability.”
Scarr, S., & McCartney, K. (1983). (See References). A groundbreaking article on how genetic differences can come to
be correlated with environmental differences over development.
Tucker-Drob, E. M., & Harden, K. P. (2012b). See reference
list. A recent study detailing a possible mechanism through
which Gene × SES interactions might operate.
Declaration of Conflicting Interests
The authors declared that they had no conflicts of interest with
respect to their authorship or the publication of this article.
Funding
This research was supported by the National Institute of Child
Health and Human Development (NICHD) Grant R21-HD069772.
D. Briley was supported by NICHD Grant T32-HD007081. The
Population Research Center at the University of Texas at Austin
is supported by NICHD Grant R24-HD042849.
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