curtis_9961.qxd 06/02/2007 9:31 AM Page 11 STANIER LECTURE 2005 A natural history of hygiene Valerie A Curtis PhD VA Curtis. A natural history of hygiene. Can J Infect Dis Med Microbiol 2007;18(1):11-14. In unpacking the Pandora’s box of hygiene, the author looks into its ancient evolutionary history and its more recent human history. Within the box, she finds animal behaviour, dirt, disgust and many diseases, as well as illumination concerning how hygiene can be improved. It is suggested that hygiene is the set of behaviours that animals, including humans, use to avoid harmful agents. The author argues that hygiene has an ancient evolutionary history, and that most animals exhibit such behaviours because they are adaptive. In humans, responses to most infectious threats are accompanied by sensations of disgust. In historical times, religions, social codes and the sciences have all provided rationales for hygiene behaviour. However, the author argues that disgust and hygiene behaviour came first, and that the rationales came later. The implications for the modern-day practice of hygiene are profound. The natural history of hygiene needs to be better understood if we are to promote safe hygiene and, hence, win our evolutionary war against the agents of infectious disease. Une histoire naturelle de l’hygiène Ouvrant la boîte de Pandore de l’hygiène, l’auteure examine son histoire évolutive ancienne et son histoire humaine plus récente. Elle trouve dans cette boîte le comportement des animaux, la saleté, le dégoût et de nombreuses maladies, ainsi qu’une illumination sur la manière d’améliorer l’hygiène. Il est suggéré que l’hygiène désigne l’ensemble de comportements que les animaux, y compris les humains, utilisent pour éviter les agents nuisibles. L’auteure avance que l’hygiène a une histoire évolutive ancienne et que la plupart des animaux adoptent ces comportements parce qu’ils sont adaptatifs. Chez les humains, la réaction à la plupart des menaces infectieuses s’accompagne de dégoût. D’un point de vue historique, les religions, les codes sociaux et les sciences ont tous apporté une motivation aux comportements d’hygiène. Cependant, l’auteure soutient que le dégoût et le comportement d’hygiène sont apparus en premier et que les motivations sont arrivées plus tard. Les conséquences sur l’hygiène moderne sont profondes. Il faut mieux comprendre l’histoire naturelle des besoins d’hygiène pour favoriser une hygiène sécuritaire et, par conséquent, gagner la lutte de l’évolution contre les agents de maladies infectieuses. Key Words: Dirt; Disgust; Evolution; Hygiene; Hygiene behaviour; Natural history; Social codes HYGIENE: FILTHY SECRETS? Hygiene is a complex Pandora’s box of a topic, full of doubtful stuff we’d rather not confront. It contains filth and disease, bugs, germs and grubby private habits. It contains ideas about obsessive cleanliness, dirty old men, and coercive states enforcing mental and racial hygiene. On the other hand, it also contains images of sparkling kitchens and bathrooms; scrubbed, perfumed and well-groomed people; and an endless array of cleaning products. Hygiene sits uneasily between filth and cleanliness; between the private and the public; and between the scientific and the moral or religious domains of society. While we all agree that hygiene is important, improving it becomes difficult if we cannot agree on what it means or understand where it comes from. So what, then, is hygiene? Ask a mother at home with her toddler and she will tell you hygiene is about cleanliness and tidiness. Ask a microbiologist and she will tell you that hygiene is about avoiding germs and disease. Ask an historian and he will tell you that hygiene first meant health, and gradually became more private and more specific over the two millennia for which we have records. An anthropologist might look at hygiene in one of two ways: the emic and the etic – the emic being the perspective of the ordinary person practicing their scrubbings and anointings, and the etic being the perspective of the scientist, objectively studying and categorizing human habits (1). In the present article, I suggest that to make sense of all these perspectives on hygiene, we need to understand its natural history. I propose that hygiene has its origins with our earliest animal ancestors, and that its evolution can be traced to the peculiar roles it plays today in our complex cultures. I suggest that hygiene has biological origins as the set of behaviours that serves to avoid infection, and that it is exhibited by most animals. I show that hygiene remains partly instinctive in humans, driven by an innate sense of the need to avoid that which disgusts. Finally, I suggest that by understanding the natural history of hygiene, we can find powerful means to improve it and, hence, help to defeat some of our ancient natural enemies – the agents of infectious disease. FOUR BILLION YEARS OF HYGIENE Far from being uniquely human, I argue that the need for hygiene arose almost as soon as animal life did. The earliest single-celled organisms to evolve represented a temptation to other organisms – parasites that wanted to use them for shelter, as a consumable resource or as a reproductive aid. Of course, we have no records of these earliest parasites, but perhaps they were akin to the modern-day phage, which are now London School of Hygiene and Tropical Medicine, London, United Kingdom Correspondence: Dr Valerie A Curtis, London School of Hygiene and Tropical Medicine, Keppel Street, London WC1E 7HT, United Kingdom. Telephone 44-207-927-2628, fax 44-207-636-7843, e-mail Val.Curtis@lshtm.ac.uk Received for publication November 8, 2006. Accepted November 13, 2006 Can J Infect Dis Med Microbiol Vol 18 No 1 January/February 2007 ©2007 Pulsus Group Inc. All rights reserved 11 curtis_9961.qxd 06/02/2007 9:31 AM Page 12 Curtis et al ubiquitous parasites on bacteria; “all the world’s a phage”, as one microbiologist put it. Lysogenic phages insert themselves into their host’s cellular machinery and use it to reproduce themselves (2). And as soon as the first thieving parasites evolved, the arms race began (3). Early unicellular life forms learned to defend themselves by building capsules that resisted attack and by developing cellular mechanisms for evicting invaders. Indeed, bacteria have evolved many specific genetic mechanisms to avoid the depredations of these viral phage parasites – for example, by phase varying the receptors that allow phage to attach to them (4). But can the deployment of cellular defences against parasitization be said to be an example of hygiene? Perhaps it might rather make sense to save the term ‘hygiene’ for behaviour or movement that is directed at avoiding disease. Which was the first animal to do this? We don’t know the answer, but because the task of avoiding parasites came early in evolution, evolving behaviours to avoid them probably started very early too. We know that Cnidarians – some of the simplest multicellular animals – can eject toxins from their body cavities in an early form of emesis (5). Simple animals, such as the nematode worm, demonstrate disease avoidance behaviours. With only 302 neurons, Caenorhabditis elegans can distinguish between innocuous and pathogenic Bacillus thuringiensis, and actively avoids the latter (6). Bullfrog tadpoles avoid other tadpoles with candidiasis (7), and lobsters avoid other lobsters with viral infections (8). Whitefish (Coregonus species) have likewise evolved mechanisms to sense and respond to the presence of Pseudomonas fluorescens, a virulent egg parasite, and to avoid it (9). Ants groom themselves to remove fungal pathogens (10), and bats groom to remove ectoparasites (11), as do other mammals, fish and birds. Some chimpanzees have been seen engaging in penile hygiene after mating (12), and mother chimps have been observed wiping the behinds of their infants (13). Birds and mammals keep their nests free of fecal material, while raccoons, badgers, lemurs and tapirs use latrines. Sheep avoid grazing near fecal remains, and one reason that reindeer and caribou migrate is to avoid parasite buildup in heavily dunged fields (14). So who taught the animals hygiene? Who taught them about the germ theory of disease and how to avoid the places where parasites are found? No one, of course. Rather, their teacher was evolution. Animals that were good at behaving in ways that avoided the ravages of micro- and macroparasites were better at passing on their ‘hygiene genes’ than those who didn’t exhibit such behaviours. Gradually, hygienic behaviours were selected for, often becoming an instinctive part of the behavioural repertoire, much like ‘flight’ or ‘freeze’ became instinctive responses to the threat of predation. So, do humans have these hygiene instincts? After a series of research projects looking into hygiene motivation around the world (eg, India, Africa, Netherlands and the United Kingdom), we found evidence for this idea (15). When interviewed about the ‘why’ of their hygiene habits, we found that people found it hard to explain their reactions to certain stimuli. Faced with feces, bodily fluids, rotten food and creepy-crawlies, people would say, “I can’t explain it – they are just yuk!” It seemed that there was a powerful sense of disgust involved, which compelled people to avoid nasty, sticky, oozing, teeming stuff. We hypothesized that disgust in 12 humans evolved to serve hygiene; in other words, to do the job of making people avoid disease. We suggested that such behaviours happen largely independent of conscious decision-making, and that disgusting cues should almost automatically lead to hygiene behaviours. THAT’S DISGUSTING! We tested our hypothesis that disgust evolved to help humans avoid disease in a Web-based experiment on the British Broadcasting Corporation’s Web site. The site showed pictures appearing in random order, and participants were asked to rate how disgusting they were on a scale of one to five. Within the series were seven pairs of photos made to be similar in appearance, but with a manipulation to heighten the disease relevance of one of the pictures. Hence, for example, a bowl of goo that was bright blue was contrasted with greeny, red-flecked goo to look like bodily fluids. An empty train was contrasted with a full one, and disgust scores for a photo of a healthy-looking person were compared with the scores for an image of the same person manipulated to look spotty and feverish. The study was completed by more than 40,000 participants from 165 countries. The results were consistent with the hypothesis; all of the images with disease relevance scored as more disgusting than those with none (16). Disgust scores declined with the age of the respondent and were significantly higher overall in women (which may be due to women’s enhanced role in child care – that is, she needs to have enough disgust to protect both herself and her dependent infant). We concluded that disgust is likely to be common to humans in all cultures, and that it serves to help us avoid those things that were associated with the risk of disease in our evolutionary past. Thus, disgust is a component of our hygiene instincts. (It is still possible to participate in the experiment by visiting .) HYGIENE IN HISTORY If hygiene is a natural function of the human psyche, originating from before we were human, then we would expect to find that, far from revelling in muck and dirt, prehistoric man would have behaved hygienically. He would have groomed himself to remove parasites and kept his living areas free from the humid wastes that can encourage their growth, survival and transmission. He would have defecated away from living areas and avoided close contact with the bodily fluids of others (except when there were overriding reasons to do so, such as when mating or caring for a child). He would have tended to avoid those of his fellows with signs of sickness (unless they were related) and also strangers (because they might have been carrying novel diseases). Hygiene behaviours do not fossilize, so evidence has to be sought elsewhere. Neanderthals apparently used seashell tweezers to pluck hair (17), and early cave paintings show beardless men, suggesting that grooming began early, perhaps to remove facial parasites. Hygiene artifacts, such as combs, are among the earliest material goods recovered. A ceremonial ivory comb in the collection of the Metropolitan Museum of New York dates back to predynastic Badarian Egypt 3200 BCE. Excavations of the earliest city states of the Indus basin dating from 3000 BC found drainage and toilet structures. Burying the dead can also be thought of as early human Can J Infect Dis Med Microbiol Vol 18 No 1 January/February 2007 curtis_9961.qxd 06/02/2007 9:31 AM Page 13 Natural history of hygiene hygiene behaviour (although there were probably further reasons for all of these practices, other than just instinctive disease avoidance). Cleansing aids have a long history. I suspect that the early cavewoman probably discovered that she could remove stubborn stains with the washed-out residue of animal fat and ash from roasting meat. However, the first recorded use of soap is in Phoenician times, although the use of oil and a scraper, known as a strigil, was a more common way of cleaning the skin in the Greek and Roman eras. Roman plumbing and toilet facilities are, of course, legendary. If early humans kept themselves and their surroundings clean, did they also avoid diseased others? An ancient Mesopotamian text shows how an exorcist explained the sickness of a patient: “He has come into contact with a woman of unclean hands … or his hands have touched one of unclean body” (18). A Babylonian letter from the 17th century BC counsels not sharing a chair, a bed or a cup with a lady suffering from a disease (19). It is not clear from these texts whether the concern was the avoidance of contagion or of immoral women. Perhaps the gut feeling of disgust provided the motive to avoid the sick, and the search for a rational explanation for why this was a good thing to do came later. Certainly, humans have continued to find rationales for what they ‘felt’ to be ‘right’ through to the present day. Sometimes, the explanations were supernatural or religious, sometimes moral, sometimes naturalistic or scientific. Purification rituals are a common feature of religions (20). In Mesopotamian times, ‘Kippuru’ was purification through the application and wiping off of a flour paste. It came to mean purification in general, as in the Hebrew word ‘Kippur’. The Laws of Manu, part of the four sacred Vedas of Hindu scripture circa 200 BC, prescribed the avoidance of the 12 impurities of the body viz “Oily exudations, semen, blood, urine, feces, the mucous of the nose, ear wax, phlegm, tears, the rheum of the eyes and sweat ...” (The Laws of Manu [5:135]). Christian morality became inextricably linked with hygiene. What was clean and ‘pure’ was what was morally right: “Wash me clean of my guilt, purify me from my sin” (Psalms 51:2). The Koran agreed: “God loves those that turn to him in repentance and strive to keep themselves clean” (2:223). Greek history relates both supernatural and naturalistic rationales for hygienic behaviour. The word ‘miasma’ originally meant ‘stain’ or sins that offended the Gods, but came to be used as a term for the foul airs and atmospheres that caused disease. It was the Greeks, however, who coined the word ‘hygiene’. Originally, the Goddess Hygieia, granddaughter of Apollo, headed a local healing cult, which spread across the Hellenic world following the plagues of 429 BC and 427 BC. Hippocrates (460 BC to 377 BC) exhorted that to stay healthy, one needed order and balance in all things and, above all, to stay away from the ‘Airs, Waters and Places’ that contained the dangerous miasmas that were responsible for disease (21). The miasma idea hopped from the Greeks via Galen and the monasteries to medieval science: “…bad, rotten and poisonous vapors from elsewhere: from swamps, lakes and chasms, for instance, and also (which is even more dangerous) from unburied or unburnt corpses – which might well have been a cause of the epidemic...” (Report on the Cause of the Plague, University of Paris Medical Faculty, 1348.) It then moved into the nineteenth century in the United Kingdom: “Disease caused by ‘…atmospheric impurities produced by decomposing animal and vegetable substances, by damp and filth, and close and overcrowded dwellings.’”(Edwin Chadwick’s ‘Report into the Sanitary Conditions of the Labouring Population of Great Britain’ – 1842.) Gradually, a modern science of hygiene developed. The work of Leeuwenhoek, Koch and Pasteur made visible the microbial agents of disease. Snow showed how they could be transmitted in populations. Florence Nightingale and Mary Seacole applied science (and a good deal of nonscientific morality, religion and common sense) to disease prevention and control, and Roger Stanier initiated molecular evolutionary studies of disease-causing organisms. Etic science moved forward, homing in on the enemy, but this was not the whole story; the direction taken by science and its progress was largely driven by innate emic wisdom. Science was following common sense, trying to prove what we always ‘knew’ was bad and disgusting, without knowing why we felt that way. The history of hygiene science could thus be said to be one of zeroing in on explanations for what we already felt in our gut and deep in the ancient animal centres of our brains. Today, we know much about the behaviour of thousands of disease-causing organisms and about human cellular defences. But we still do not understand human disease avoidance behaviour – a strange omission because changing behaviour is surely our most potent way of preventing disease. THE HYGIENE CHALLENGE TODAY Does the insight that hygiene behaviour is driven not by rationality, but by deep and ancient urges within us, which are not entirely under our conscious control, have any implications for hygiene today? While over two million children still die from infectious intestinal diseases every year (22), the need to find solutions is pressing. We have proposed that it is possible to harness the deeper motivations that drive behaviour in the service of hygiene. While health education – that is, teaching people about germs – may play a role in eliciting safer behaviour, it may be more effective to build on our already hard-wired instincts to avoid contamination (23). In a recent attempt to reduce the toll of diarrheal diseases among children in Ghana by getting people to wash their hands with soap (24), we developed a campaign to make people feel that not washing hands with soap was disgusting. The television advertisement, for example, showed a strange stain on a mother’s hands after coming from the toilet, which was then transferred to her child’s food. Despite not using rational arguments about germs or disease, the campaign led to 41% more Ghanaians reporting washing hands with soap in national samples (25). (The advertisement can be viewed at .) Recognizing the importance of ancient, evolved motivations may help us to zero in on more effective public health interventions designed to improve hygiene. (Our explorations of hygiene Can J Infect Dis Med Microbiol Vol 18 No 1 January/February 2007 13 curtis_9961.qxd 09/02/2007 3:35 PM Page 14 Curtis also led us to other ancient motives for cleanliness: to care for and nurture children, to avoid social disapprobation, to do what others are doing, to be more attractive. Such motives can also be harnessed in the service of hygiene promotion.) Although we humans are proud of our rationality, we cannot explain the whole of our behaviour through conscious, logical calculation. Because human symbolic thought arrived relatively recently and the understanding of germs is very recent (and not yet ubiquitous), hygienic behaviour cannot be explained purely as a conscious response to disease threats. Rather, the scrubbings, purgings, tidyings and separations we make are a product of our natural history. They have their origins with our most ancient ancestors, the first animals, who practiced hygiene to avoid being eaten from within by parasites. These animals knew nothing about the germ theory of disease. In the same way, societies throughout history have been hygienic without the discoveries of Pasteur or the molecular biologists. Science continues to zero in on, to make more precise, what we ‘feel’ to be right: dirt causes disease. But as a species, we are naturally hygienic – in fact, we ‘knew’ that all along. ACKNOWLEDGEMENTS: The author thanks Robert Aunger for collaboration in this work, Adam Biran for excellent comments, and Miguel Rubio-Godoy for zoological references. The author also thanks Dr Morris Goldner, Visiting Professor, Brock University, and President of the Stanier Institute/Institut Stanier, for organizing the Stanier Lectures and Symposia. Gratitude is expressed to Shawna Bourne CPHI(C), Stanier Resident Writer and Editor, for organizing the Stanier Publications. REFERENCES 1. Harris M. Cultural Anthropology. New York: Harper and Row, 1983. 2. Hendrix RW, Lawrence JG, Hatfull GF, Casjens S. The origins and ongoing evolution of viruses. Trends Microbiol 2000;8:504-8. 3. Dawkins R. The Selfish Gene. New York: Oxford University Press, 1976. 4. Zaleski P, Wojciechowski M, Piekarowicz A. The role of Dam methylation in phase variation of Haemophilus influenzae genes involved in defence against phage infection. Microbiology 2005;151:3361-9. 5. Rubio-Godoy M, Aunger R, Curtis V. Serotonin – a link between disgust and immunity? Med Hypotheses 2007;68:61-6. 6. Schulenburg H, Muller S. Natural variation in the response of Caenorhabditis elegans towards Bacillus thuringiensis. Parasitology 2004;128:433-43. 7. Kiesecker JM, Skelly DK, Beard KH, Preisser E. Behavioral reduction of infection risk. Proc Natl Acad Sci USA 1999;96:9165-8. 8. Behringer DC, Butler MJ, Shields JD. Avoidance of disease by social lobsters. Nature 2006;441:421. 9. Wedekind C. Induced hatching to avoid infectious egg disease in whitefish. Curr Biol 2002;12:69-71. 10. Hölldobbler B, Wilson EO. The Ants. Cambridge: Harvard University Press, 1990. 11. Hofstede H, Brock Fenton M. Relationships between roost preferences, ectoparasite density, and grooming behaviour of neotropical bats. J Zool 2005;266:333-40. 12. O’Hara SJ, Lee PC. High frequency of postcoital penis cleaning in Budongo chimpanzees. Folia Primatol (Basel) 2006;77:353-8. 14 13. Goodall J. The Chimpanzees of Gombe. Cambridge: Harvard University Press, 1986. 14. Gunn A, Irvine RJ. Subclinical parasitism and ruminant foraging strategies: A review. Wildl Soc Bull 2003;31:117-26. 15. Curtis VA, Biran A. Dirt, disgust, and disease. Is hygiene in our genes? Perspect Biol Med 2001;44:17-31. 16. Curtis V, Aunger R, Rabie T. Evidence that disgust evolved to protect from risk of disease. Proc Biol Sci 2004;271(Suppl 4):S131-3. 17. Musgrave JH. How dextrous was Neanderthal man? Nature 1971;233:538-41. 18. Burkert W. Greek Religion. Oxford: Blackwell, 1985. 19. Biggs R. Medicine in ancient Mesopotamia. Hist Sci 1969;8:94-105. 20. Douglas M. Purity and Danger: An Analysis of Concept of Pollution and Taboo. London: Routledge, 1966. 21. Hippocrates. Hippocratic Writings. Harmondsworth: Penguin, 1983. 22. World Health Organization. World Health Report 2002. (Version current at January 11, 2007). 23. Curtis V. Hygiene: How myths, monsters and mothers-in-law can promote behaviour change. J Infect 2001;43:75-9. 24. Curtis V, Cairncross S. Effect of washing hands with soap on diarrhoea risk in the community: A systematic review. Lancet Infect Dis 2003;3:275-81. 25. Curtis VA, Garbrah-Aidoo N, Scott B. Masters of marketing: Bringing private sector skills to public health partnerships. Am J Public Health 2007. (In press) Can J Infect Dis Med Microbiol Vol 18 No 1 January/February 2007 660 PUBLIC HEALTH PAST AND PRESENT Dirt, disgust and disease: a natural history of hygiene Valerie A Curtis ................................................................................................................................... J Epidemiol Community Health 2007;61:660–664. doi: 10.1136/jech.2007.062380 Hygiene has been studied from multiple perspectives, including that of history. I define hygiene as the set of behaviours that animals, including humans, use to avoid infection. I argue that it has an ancient evolutionary history, and that most animals exhibit such behaviours because they were adaptive. In humans, the avoidance of infectious threats is motivated by the emotion of disgust. Intuition about hygiene, dirt and disease can be found underlying belief about health and disease throughout history. Purification ritual, miasma, contagion, zymotic and germ theories of disease are ideas that spread through society because they are intuitively attractive, because they are supported by evidence either from direct experience or from authoritative report and because they are consistent with existing beliefs. In contrast to much historical and anthropological assertion, I argue that hygiene behaviour and disgust predate culture and so cannot fully be explained as its product. The history of ideas about disease thus is neither entirely socially constructed nor an ‘‘heroic progress’’ of scientists leading the ignorant into the light. As an animal behaviour the proper domain of hygiene is biology, and without this perspective attempts at explanation are incomplete. The approaches of biological anthropology have much to offer the practice of cultural history. ............................................................................. In every street the pipes gushed out where decaying rat carcases drank everything in, tails dangling and whiskers bristling with greenish lumps. Bellies in the air, they floated amid apple peels, asparagus stalks and cabbage cores…it was like a vast infection of tooth decay, like the flatulence of a rotting stomach, like the emanations of a man who has drunk too much, like the dried sweat of rotting animals, like the sour poison of a bedpan…this avalanche of excretions tumbling down the length of the purulent streets…let off its nocturnal fragrances.1 (p 253) ........................ Correspondence to: Valerie A Curtis, The Hygiene Centre, London School of Hygiene & Tropical Medicine, Keppel St, London WC1E 7HT, UK; Val.curtis@lshtm.ac.uk Accepted 15 May 2007 ........................ www.jech.com This extract from a satirical article which appeared in Le Figaro in 1880 captures something of the disgust the citizens of Paris must have felt for the state of their streets at that time. Barnes, in his new book on the great stinks of Paris, uses this description to illustrate what he claims to be a major historical paradox. How can it be, he asks, that this disgust, ‘‘which was shaped by changing cultural norms and rules through history, can manifest itself in such a gut-level seemingly unconscious way’’?1 Historians and anthropologists have long wrestled with the question of why and how certain objects and events come to be classed as dirty and disgusting and what, if anything, is their connection with disease. Some have taken the ‘‘master narrative’’ approach in which heroic figures such as Hippocrates, Chadwick, Snow, Pasteur and Koch led an ignorant public from filth into the hygienic light of scientific rationalism.2 3 Douglas rejected such thinking as ‘‘medical materialism’’, proposing instead that the objects and events that societies classed as dirty were those that did not fit the local cosmology, and hence had to be rejected: Dirt then, is never a unique, isolated event. Where there is dirt there is a system. Dirt is the by-product of a systematic ordering and classification of matter, in so far as ordering involves rejecting inappropriate elements.4 Modern historians of science such as Barnes and Tomes offer a ‘‘revisionist’’ view which takes a relativistic approach to the social production of ideas. For example, Tomes proposes that we should see ideas, such as the germ theory of disease, as not as having an ontological life of their own but as social constructions, embedded within local structures of meaning.5 For them, like Douglas, dirt is a cultural construction. In this paper I argue that medical historians, whether materialist or revisionist, are missing an important point. There is a link between dirt, disgust, hygiene and disease, but it is a link that predates history, that predates science and culture, that even predates Homo sapiens. Disgust has a long evolutionary history; the reason it is part of our psyche is neither primarily cultural nor historical, but biological. Animals that were equipped with behavioural tendencies which led them to avoid the objects and events that were associated with the risk of disease gained an adaptive advantage; hence any genes that favoured hygienic behaviour tended to outperform those that did not. Whilst the specifics of what we find disgusting are, of course, shaped by experience and culture, there is an overall biological pattern to our revulsions. Disgust of dirt is a part of human nature. To tell this alternative, biological history of dirt, disease and hygiene, I take a long view. I describe the evolutionary origins of disease avoidance behaviour in animals and present evidence as to the role of disgust in the prevention of disease in humans. I then take examples from the historical literature to make a first, necessarily brief, attempt at weaving this fact of our nature back into historical narrative. The approach I take is neither materialist nor revisionist, but epidemiological. If Dirt, disgust and disease ideas spread best in susceptible hosts, then being equipped with disgust makes us susceptible to culturally transmitted ideas, such as the value of purity and hygiene and the dangers of dirt, miasma and germs. I conclude that, from this perspective, Barnes’ paradox is not a paradox at all. Dirt and disgust are both gut feeling and cultural construction – two related facets of the natural history of hygiene. FOUR BILLION YEARS OF HYGIENE The story of hygiene begins, not with Homo sapiens, but with the earliest animals. Animals need nutrients in order to survive; they can get those nutrients either by eating plants or other animals.6 Other animals are tempting targets because they are concentrated sources of nutrition. An animal which eats an animal smaller than itself is a predator. An animal that eats an animal larger than itself is a parasite.7 Animals pursuing and eating each other lead to evolutionary arms races, where each species attempts to outwit the other with new adaptations.8 We have no record of the earliest parasites, but perhaps they were akin to the modern-day phages, which are viral parasites of bacteria and the most common life form on the planet. Phage insert themselves into their bacterial hosts’ cellular machinery and use it to reproduce themselves.9 Bacteria defend themselves with capsules that resist attack and cellular responses for evicting invaders. Indeed, much of the modern bacterial genome concerns defences against phages.10 Whilst all animals have physiological defences against parasite invasion, this can hardly be called ‘‘hygiene’’. We have reserved the term for behaviours which help to avoid the risk of being invaded by parasites, whether these are microscopic, such as viruses and bacteria, or macroscopic, such as helminths and scabies mites.11 Hygienic behaviour can thus include grooming to remove ectoparasites, avoiding contact with potentially parasitised others, and avoiding or removing substances or other species in the environment that are likely to harbour pathogenic parasites.12 Tracing the early history of hygiene is difficult because behaviours do not fossilise. However, perhaps our earliest common ancestor with the roundworms who lived some 590 million years ago13 was hygienic. Their extant cousins, the wellstudied nematode worms, Caenorhabditis elegans demonstrate disease avoidance behaviour. With only 302 neurons, they can detect the presence of pathogenic Bacillus thuringiensis in the environment and avoid it.14 Ants are hygienic: they groom themselves to remove fungal pathogens and dispose of diseased and dead conspecifics.15 16 Bees remove dead and diseased brood, defecate away from the nest and employ antibacterial compounds to keep nests free of parasites.17 Caribbean spiny lobsters (Panulirus argus) avoid other lobsters with viral infections.18 It is likely that all vertebrates exhibit hygiene behaviour. Bullfrog (Rana catesbeiana) tadpoles avoid other tadpoles with signs of candidiasis,19 and whitefish, Coregonus sp, have evolved the ability to respond to the presence of Pseudomonas fluorescens, a virulent egg parasite, by avoiding it.20 Bats groom to remove ectoparasites,21 as do most other mammals, fish and birds. Birds and mammals keep their nests free of faecal material, whilst racoons, badgers, lemurs and tapirs use latrine sites. Sheep avoid grazing near faecal remains, and one reason that reindeer and caribou migrate is to avoid parasites in heavily dunged fields.22 Some chimpanzees have been seen engaged in penile hygiene after mating23 and mother chimps have been observed wiping the behinds of their infants after they have defaecated.24 Clearly, no one taught these animals microbiology or parasitology. How, then, could such behavioural tendencies have arisen? The long, gradual process of evolution was their teacher – quite simply, the genes of animals that failed to 661 defend themselves effectively against disease were selected out of the gene pool, while those that contributed to good strong hygiene instincts survived and multiplied disproportionately well. Appropriate hygienic behaviour thus became typical to each species in its niche. Do humans have these hygiene instincts tooi? After a series of research projects looking into hygiene motivation around the world (India, Africa, Netherlands, UK) we found evidence for this idea.11 When interviewed about the ‘‘why’’ of their hygiene habits, people found it hard to explain their reactions. Faced with faeces, bodily fluids, rotten food and slimy worms most people would say that they couldn’t explain, they just felt they were ‘‘Yuk!’’. It seemed that there was a powerful feeling of disgust involved, impelling people to avoid filthy, sticky, oozing, teaming matter. We hypothesised that at its core disgust is what humans call the urge to avoid disease-relevant stimuli. We suggested that such behaviours happen largely independent of conscious decision-making, and that the perception of a disgusting cue should almost automatically produce a hygienic reaction. THAT’S DISGUSTING! We tested the hypothesis that disgust evolved to motivate humans to avoid disease in a web-based experiment placed on the BBC’s website http://www.bbc.co.uk/science/humanbody/ mind/surveys/disgust/. The site asked people to score how disgusting they found a series of photos. Within the series we randomly mixed seven sets of pairs of images, made to be similar in appearance, but in each case one of the pictures contained a disease threat, whilst the other did not. Hence, for example, the results for a bowl of bright blue viscous liquid were compared with those for a greeny, red-flecked version, which looked somewhat like bodily fluids. A clean burn was contrasted with an infected wound. An empty train was compared with a full one. Disgust scores for a photo of a healthy-looking person were compared with the scores for an image of the same person manipulated to look spotty and feverish. The study was completed by more than 40 000 participants from 165 countries. The results were consistent with our hypothesis: all of the images with disease relevance scored significantly higher for disgust than those with none.26 Disgust sensitivity scores declined with the age of the respondent and were significantly higher overall in females (which may be due to their role in child care). We concluded that disgust is probably common to all humans in all cultures, and that it serves to help us to avoid those things that were associated with risk of disease in our evolutionary past. Disgust is thus the name we give to the motivation to behave hygienically. It is thought that this basic emotion became extended at some point in evolutionary history – other studies have found evidence for a further domain of disgust where immoral acts and associations can occasion revulsion.11 27 28 HYGIENE IN HISTORY If producing hygienic behaviour is a natural function of the human psyche, a psychological predisposition designed by evolution to keep us safe from disease that originates from before we were human, and from before the dawn of culture, then we would expect to find that, unlike his caricature, prehistoric man would have behaved hygienically. He would have groomed himself to remove parasites and kept his living i Note that the definition of the word ‘‘instinct’’ is much debated amongst biologists and philosophers of biology (see Mameli25 for a recent contribution). Here we take it to mean automated behaviours produced by specific cues in the environment that do not require the involvement of higher level processes such as cognition. www.jech.com 662 areas free of the humid wastes that can harbour them. He would have defaecated away from his shelter and avoided close contact with the bodily fluids of others (except when there were overriding reasons to do so, such as when mating or caring for a child). He would have tended to avoid those of his fellows exhibiting signs of sickness (unless they were related) and dead bodies, and also strangers (because they might carry novel pathogens). Evidence of early hygiene behaviour among humans is hard to come by. The earliest signs of the interment of the dead date from the middle Palaeolithic era. Neanderthals used seashell tweezers possibly to pluck hair29 and early cave paintings show beardless men, suggesting that grooming began early, perhaps to remove facial parasites.30 Hygiene artefacts are amongst the earliest material goods recovered; for example, an ivory comb in the collection of the Metropolitan Museum of New York dates back to predynastic Badarian Egypt of 3200 BCE. Excavations of the earliest city states of the Indus basin dating from 3000 BC found drainage and toilet structures. Cleansing aids have a long history. Early cavewoman may have discovered she could remove stubborn stains with the washed out residue of animal fat and ash from roasting meat. However, the first recorded use of soap is from Babylonian times, although the use of oil and a scraper, known as a strigil, was a more common way of cleaning the skin in the Greek and Roman eras. Roman plumbing and toilet facilities are well documented.31 If early humans kept themselves and their surroundings clean, did they also avoid diseased others, as other vertebrates do? An ancient Mesopotamian text shows how an exorcist explained the sickness of a patient: ‘‘He has come into contact with a woman of unclean hands…or his hands have touched one of unclean body’’.32 A Babylonian letter of the 17th century BC counsels not sharing a chair, a bed or a cup with a lady suffering from a disease.33 Perhaps the gut feeling of disgust provided the motive to avoid the sick, giving impetus to the search for a rational explanation. Are these early hygiene behaviours a product of nature or culture? I have argued that hygiene predates culture; however, once humans evolved the cognitive ability to use symbols and language to make representations, then it became possible for ideas about hygiene to begin to accumulate and to begin to influence human behaviour. Curtis purification through the application and wiping off of a flour paste. It came to mean purification in general, as in the Hebrew word ‘‘Kippur’’. The idea that it was good to remove disgusting matter – pollution – made intuitive sense. Powerful priesthealers related their feats of healing based on this principle, which gave authority to the idea, helping it to spread widely. The intuition that polluting matter should be removed or avoided can be found woven throughout religious and cultural history.4 The Laws of Manu, part of the four sacred Vedas of Hindu scripture of circa 200 BC, prescribed the avoidance of the 12 impurities of the body, viz: Oily exudations, semen, blood, urine, faeces, the mucous of the nose, ear wax, phlegm, tears, the rheum of the eyes and sweat... (135) The intuition that dirt was bad helped carry filth into the realm of morality. Many biblical passages make this link, for example: Wash me clean of my guilt, purify me from my sin (Psalms 51:2) The Koran agreed: God loves those that turn to him in repentance and strive to keep themselves clean (2:223) The Greek word ‘‘miasma’’ grew from similar religious and moral origins into a naturalistic theory of disease. Originally meaning ‘‘stain’’ or pollution of sin which offended the gods, it came to be used as a term for the foul airs and atmospheres that were thought to cause disease.38 Hippocrates (460–377 BC) exhorted that to stay healthy one needed to stay away from the airs, waters and places that contained dangerous vapours or miasmas.39 The idea hopped from the Greeks via Galen and the monasteries to medieval science: …bad, rotten and poisonous vapours from elsewhere: from swamps, lakes and chasms, for instance, and also (which is even more dangerous) from unburied or unburnt corpses – which might well have been a cause of the epidemic...40 AN EPIDEMIOLOGY OF IDEAS Sperber suggests that the process by which ideas or mental representations spread through society is epidemiological.34 Some representations spread better than others and hence are more widely distributed, because human cognitions and communicative abilities work better on some ideas than on others. The ideas that tend to be favoured in this way, all other things being equal, are those which make intuitive sense.35 So, for example, beliefs that nasty, steaming, humid, smelly, excreted stuff is bad for you might spread much more easily than ideas proposing the opposite. Of course, as in epidemiology, not all other things are equal; host susceptibility is not the only factor that determines spread. Other factors include how well a new idea sits with an existing belief system, how well it fits in with what people have already observed and whether the source of the new idea is trusted or authoritative.34 36 37 Can we trace this epidemiology of infectious ideas in the early recorded history of hygiene? In the Middle Eastern contagion examples above, the belief that disease could be passed on would have come from observation. However, the idea was believable because of the intuition that the sick were to be avoided. A similar example is provided by purification rituals. In Mesopotamian times Kippuru was a healing ritual of www.jech.com And thence into the 19th century in the UK: Disease caused by…atmospheric impurities produced by decomposing animal and vegetable substances, by damp and filth, and close and overcrowded dwellings.2 The belief that miasma caused disease was an extremely infectious idea – one that continued to inhabit the population of Western Europe for over 2000 years. Behind this belief again lay disgust – the intuitive aversion to bad smells from putrid, fetid, damp environments. The intuition hardened into belief through an accumulation of observations – that people did, indeed, often catch fevers in swampy environments. However, by the mid 19th century in Europe new observations began to accumulate that made it harder to believe that disease was caused by breathing bad air. Agriculturalist Justus van Leibig’s experimentation with the decomposition of vegetable matter led him to propose that: …disease was due to a spreading internal rot…that came from an external rot…that could be transferred to others.41 Dirt, disgust and disease For a short while the zymotic theory of disease gained acceptance.41 The new idea was that disease was caused, not by bad air, but through a chemical process involving direct contact. The theory had the characteristics of an infective idea because it was intuitively appealing, as rot was certainly disgusting. It also fitted with the existing idea and experience of rot spreading in vegetables. Reports of putrefaction and sepsis creeping their way through diseased flesh that were provided by the practitioners of the increasingly invasive surgery and dissection of the mid 1800s42 added weight to the new explanation of how the disgusting caused disease. There is much debate over the details of how and when the germ theory of disease took hold. A key development was, of course, the microscope. Yet it was more than 300 years after Leewenhoek demonstrated the teeming animalcules in the white matter between his ‘‘usually very clean’’ teeth,3 that belief in germs became an established norm in Western medical discourse. The idea that living organisms were responsible spread in fits and starts through Europe and America1–3 42 43 over the second half of the 19th century. Germ theory had some, but not all, of the components of an infectious idea. It did make intuitive sense; if disease was being caused by an invasion of the body by living organisms this was exquisitely disgusting, and hence to be avoided at all costs. However, since people could have no direct experience with germs it remained for the scientists to provide convincing evidence. Perhaps it was only the wide publication of pictures of microbes from the labs of Koch, Pettenkoffer, Yersin and their colleagues in the scientific and then the popular press, that germ theory really began to take hold. With the dawn of vaccination and antimicrobials, it became hard to deny the testimony of respected scientists that germs were responsible for at least some types of infectious disease.3 Germ theory spread unevenly across much of the world, sometimes to replace and sometimes to live side-by-side or hybridise with local beliefs about the origins of disease. A Chinese text of 1911 on germ theory taught that tuberculosis was caused by ‘‘xijun’’, which translates literally as ‘‘tiny fungus’’ or ‘‘mould’’. TB is still commonly described as being due to ‘‘ji laochong’’, or wasting worms.44 In modern Hindi germs are ‘‘kitanu’’, which means ‘‘little insect’’, and in Dioula (Burkina Faso) they are ‘‘banakisse’’ or seeds of disease. But in Burkina Faso germ theory has not banished other ideas about disease. The intuition that dirt causes disease is, of course, well established. However, diarrhoea in children is also thought to be caused by contamination of the mothers’ milk by sperm if sexual relations have been resumed post-partum, and by worms and by feeding the wrong food; however, only ‘‘toubaboukonoboli’’ or ‘‘white’s diarrhoea’’ is thought to be caused by germs.45 This epidemiologic spread of Western ideas into other cultures is underpinned by the same intuition – that it could not be good for one to be invaded by disgusting tiny insects, fungus or seeds (germs). Pictorial representations of germs are now much more common, making them ‘‘observable’’ (for example as cartoons in advertisements for Lifebuoy soap screened throughout India in recent years). Finally, germ theory gained ground because it was taught by respected others who were known to have real cures for disease, in this case, the powerful colonisers, with their science and their microscopes. DISCUSSION: DIRT AS A CULTURAL CONSTRUCTION I have suggested that hygiene and disgust originated well before culture and history; hence it would be wrong to think of disgust as being entirely a cultural construction. It may be just as useful to study how a biological capacity for disgust has 663 influenced cultures as to explore how cultures have influenced what people find disgusting. In the end, why does it matter whether disgust is innate, a cultural construction or both, as I would argue? In my view the issue is fundamental, firstly for our intellectual traditions, both in history and in the humanities, and secondly, due to its practical and policy implications. The work of Mary Douglas on the cultural construction of purity and pollution has inspired a school of historical and cultural investigation that insists that cultures are what create dirt, taboo and transgression. Douglas has the local cosmology, or world order, coming first, with dirt as its product. For her dirt is matter out of place, an anomaly that has to be banished because it threatens the order of the system.4 Cultural commentators on filth such as Cohen46 and Miller47 continue in the Douglasian tradition, puzzling over the same paradox as Barnes: how can something as visceral as disgust be produced by history and culture? Yet if the dirty is what disgusts us, then this is surely wrong: dirt arose before culture and history, and therefore cannot just be its product. Tomes, Barnes and others also follow Douglas in setting up an opposition between a materialist reading of the history of ideas about pollution, hygiene and disease and a revisionist one. For them, contingent local processes of idea assimilation provide a better explanatory framework than the grand progress of science. In this account of the natural history of hygiene I have rather proposed an epidemiological process of the spread of ideas which admits multiple determinants. Innate psychological susceptibility to ideas linking disgust and hygiene provide one determinant. Whilst it may not exactly have been a grand progress, the scientific method of observation and falsification did allow real advances in understanding, which influenced and continues to influence, the content of culture. However, we have also seen contingent local processes of assimilation of new ideas adapting to local cultures. In this sense, ideas about hygiene and disease are, indeed, cultural constructs. Taken together, these three factors provide a useful, and to some extent, testable set of hypotheses about the advance of ideas. It may be argued that this is an overly simplistic rendition of the history of a complex set of ideas about dirt, disgust, disease and hygiene. In this short piece I have proposed a small set of key determinants of historical processes, when of course there are many other factors. Geography, genetics, demography, environment and technology are amongst many further determinants of the patterns of ideas we are seeking to explain.48 Yet every scientific endeavour requires making hypotheses and mechanisms explicit, which requires simplification.49 If we accept a role for innate psychology in the content of our culture, then there are important practical implications as well as intellectual ones. If disgust arose to help us deal with ancestral disease threats, then it may not be a good guide to the best way to avoid disease in our modern environments. When evolutionarily novel infections such as HIV/AIDS arise, our evolved responses to avoid the infection may mislead. For example, quarantine and the avoidance of physical contact were intuitive but not useful responses to a sexually transmitted disease with a long latency period. Those who care for others find dealing with bodily fluids difficult and disgusting and find little support, since the topic is still taboo. Recognising that such reactions are a part of our nature could help towards appreciating the emotional labour involved – and be an important step towards more humane caring and caring for carers. Politicians have a sorry history of making capital by exploiting a tendency to xenophobia – disgust of the outsider.47 Regular epidemics of panic about contamination in food also www.jech.com 664 Curtis What this paper adds N N N I propose that disgust of the dirty predates history and hence cannot be its product I suggest that patterns in the history of ideas can be understood using the methods of epidemiology. Host susceptibility to intuitive ideas, such as the undesirability of the disgusting, is an important component in the history of ideas about the causation of disease. Policy implications N N N Disgust tends to guide theorising in public health but can mislead, for example, quarantine is not a good response to HIV/AIDS. Dealing with disgust is a part of the emotional labour involved in the caring professions and should be recognised as such. Scientists and policy makers need to be aware that ancient psychological tendencies that were appropriate for early society are not always a good guide to decisionmaking in today’s environments. owe more to an innate disgust of contamination than any rational science of relative risk. Scientists need to understand how such ancient psychological factors influence the historical progress of science, and can impact on their own beliefs. In the end I am arguing for two things, for a return of human nature to a legitimate place in the humanities,50 and for history to embrace biology and its methods, including those of biological anthropology and epidemiology, and hence take its natural place as one of the life sciences.51 52 ACKNOWLEDGEMENTS Thanks to Unilever PLC for supporting the work of the Hygiene Centre, to Robert Aunger for collaboration in this work, Adam Biran and Mark Bradley for excellent comments, and Miguel Rubio Godoy for zoological references. Competing interests: None declared. REFERENCES 1 Barnes DS. The great stink of Paris and the nineteenth century struggle against filth and germs. Baltimore, MA: Johns Hopkins University Press, 2006. 2 Rosen G. A history of public health. Baltimore, MA: Johns Hopkins University Press, 1951. 3 Waller J. The discovery of the germ. Cambridge, UK: Icon Books, 2002. 4 Douglas M. Purity and danger: an analysis of the concepts of pollution and taboo. London: Routledge and Kegan Paul, 1966. 5 Tomes N, Warner J. Rethinking the reception of the germ theory of disease: comparative perspectives. J Hist Med 1997;52:7–16. 6 McNeill W. Plagues and peoples. London: Penguin Books, 1976. 7 Zimmer C. Parasite Rex. New York: Touchstone, 2000. 8 Ridley M. The red queen: sex and the evolution of human nature. London: Viking, 1993. 9 Hendrix RW, Lawrence JG, Hatfull GF, et al. The origins and ongoing evolution of viruses. Trends Microbiol 2000;8:504–8. www.jech.com 10 Zaleski P, Wojciechowski M, Piekarowicz A. The role of Dam methylation in phase variation of Haemophilus influenzae genes involved in defence against phage infection. Microbiology 2005;151:3361–9. 11 Curtis VA, Biran A. Dirt, disgust and disease: is hygiene in our genes? Perspect Biol Med 2001;44:17–31. 12 Hart BL. Behavioural adaptations to pathogens and parasites: five strategies. Neurosci Biobehav Rev 1990;14:273–94. 13 Dawkins R. The ancestor’s tale. London: Weidenfeld & Nicolson, 2004. 14 Schulenburg H, Muller S. Natural variation in the response of Caenorhabditis elegans towards Bacillus thuringiensis. Parasitology 2004;128:433–43. 15 Hölldobbler B, Wilson EO. The ants. Cambridge, MA: Harvard University Press, 1990. 16 Franks NR, Hooper J, Webb C, et al. Tomb evaders: house-hunting hygiene in ants. Biol Lett 2005;1:190–2. 17 Visscher PK. Hygiene in honey bee colonies. Am Bee J 1983;123:511–3. 18 Behringer DC, Butler MJ, Shields JD. Avoidance of disease by social lobsters. Nature 2006;441:421. 19 Kiesecker J, Skelly D, Beard K, et al. Behavioral reduction of infection risk. Proc Natl Acad Sci USA 1999;96:9165–8. 20 Wedekind C. Induced hatching to avoid infectious egg disease in whitefish. Curr Biol 2002;12:69–7. 21 Hofstede H, Brock Fenton M. Relationships between roost preferences, ectoparasite density, and grooming behaviour of neotropical bats. J Zool Lond 2005;266:333–40. 22 Gunn A, Justin Irvine R. Subclinical parasitism and ruminant foraging strategies-a review. Wildlife Soc Bull 2003;31:117–26. 23 O’Hara S, Lee P. High frequency of post-coital penis cleaning in Budongo chimpanzees. Folia primatol (Basel) 2006;77(5):353–8. 24 Goodall J. The chimpanzees of Gombe. Cambridge, MA: Harvard University Press, 1986. 25 Mameli M, Bateson P. Innateness and the sciences. Biol Philos 2006;21:155–88. 26 Curtis V, Aunger R, Rabie T. Evidence that disgust evolved to protect from risk of disease. Proc R Soc Lond B 2004;271:S131–3. 27 Haidt J, Rozin P, McCauley C, et al. Body, psyche, and culture: the relationship of disgust to morality. Psychol Developing Soc 1997;9:107–31. 28 Rozin P, Haidt J, McCauley CR. Disgust. In: Lewis M, Haviland JM, eds. Handbook of emotions. New York: Guilford Press, 2000:637–53. 29 Musgrave J. How dextrous was Neanderthal man? Nature 1971;5321:538–41. 30 Pagel M, Bodmer W. A naked ape would have fewer parasites. Proc R Soc Lond B (Suppl) 2003;270:S117–S9. 31 Scobie A. Slums, sanitation and mortality in the Roman world. Klio 1986;68:399–433. 32 Burkert W. Greek religion. Oxford: Blackwell, 1985. 33 Biggs R. Medicine in ancient Mesopotamia. Hist Sci 1969;8:94–105. 34 Sperber D. Explaining culture: a naturalistic approach. Oxford: Blackwell, 1996. 35 Haidt J, Joseph C. Intuitive ethics: how innately prepared intuitions generate culturally variable virtues. Dædalus 2004;133:55–66. 36 Aunger R. The Electric meme. New York: The Free Press, 2002. 37 Boyer P. Religion explained. London: Heinemann, 2001. 38 Parker R. Miasma: pollution and purity in early Greek religion. Oxford: Clarendon Press, 1983. 39 Hippocrates. Hippocratic writings. Harmondsworth: Penguin, 1983. 40 University of Paris Medical Faculty. Report on the cause of the plague. Paris: University of Paris, 1348. 41 Hamlin C. Providence and putrefaction: Victorian sanitarians and the natural theology of health and disease. Victorian Stud 1985;28:381–411. 42 Worboys M. Spreading germs: disease theories and medical practice in Britain 1865–1900. Cambridge: Cambridge University Press, 2000. 43 Tomes N. The gospel of germs, men, women and the microbe in American life. Cambridge, MA: Harvard University Press, 1998. 44 Andrews B. Tuberculosis and the assimilation of germ theory in China, 1895– 1937. J Hist Med 1997;52:114–57. 45 Kanki B, Curtis V, Mertens T, et al. Des croyances au comportements: diarrhée et pratiques d’hygiène au Burkina Faso. Cahiers Santé 1994;4:335–9. 46 Cohen WA, Johnson R. Filth: dirt, disgust and modern life. Minneapolis, MN: University of Minnesota Press, 2005. 47 Miller WI. The anatomy of disgust. Cambridge, MA: Harvard University Press, 1997. 48 Diamond J. Guns, germs and steel. London: Jonathan Cape, 1997. 49 Rosenberg A. Philosophy of social science. Boulder, CO: Westview Press, 1988. 50 Pinker S. The blank slate: the modern denial of human nature. London: Allen Lane, 2002. 51 Wilson EO. Consilience. New York: Knopf, 1998. 52 Burkert W. The creation of the sacred: tracks of biology in early religions. Cambridge, MA: Harvard University Press, 1996. Essay Writer: ___________________________ Peer Reviewer: __________________________ Peer Review for Argumentative Essays Directions: Use complete sentences for your responses and refer frequently to the essay. 1. Underline or highlight the writer’s overall claim or thesis statement and discuss its effectiveness. 2. How does the introduction get the reader interested in the problem being presented? 3. What are the writer’s points in support of the main claim? After listing them, circle or highlight the one you find most convincing and briefly explain why. a. d. b. e. c. f. 4. What counterarguments does the writer address? If you don’t spot any, suggest at least one possible objection to the writer’s position. (Be specific.) 5. What concessions (if any) does the writer make? If you don’t spot any, suggest at least one point the writer might want to qualify or concede. (Be specific.) 6. What makes the writer’s line of reasoning clear? If it’s not, what logical fallacies hurt the argument and explain why? Be specific. 7. Describe one of your experiences with this subject that the writer might find interesting or relevant. a. b. c. 8. How does the conclusion effectively close the argument? 9. List the sources that are used in the essay, and circle the direct quotations or references used from each source. Which supporting reference seems most effective and credible and which least? a. d. b. e. c. f. 10. As you review the essay, carefully check the format of quotations and other references to outside sources. Give an example of one that seems incorrect. This is very important. 11. Does every source listed above (see question 9) appear on the Works Cited page? Are those entries arranged in alphabetical order on a separate page? If not, make a note. 12. List 3 strong points about the essay and explain your reasons. 13. List 3 ways to improve this essay and tell why. Chunk Paragraph Template and Examples 1. Template: Follow this pattern to complete a well written paragraph: 1. T.S. - Topic Sentence 2. L.I. followed by a substantial 3. C.D. – Lead in and concrete detail 4. C.M. – Commentary 5. C.M. – Commentary 6. C.S. – Concluding sentence 2. Step by Step THE QUOTE Begin by deciding which quote to use as the center of your CHUNK paragraph. Make sure the quote is meaningful and substantial. Remember, as instructed in class, start from the middle out. The quote is the center (literally and figuratively) of your paragraph. 3. STEP BY STEP The Lead In The function of the lead in is to put the quote in context. The lead in may inform the reader of a few of the who, what, when, where, why and/or how of the quote. The lead in is placed before the quote and ends with a comma. (See example) 4. STEP BY STEP The Topic Sentence Now that you’ve chosen a meaningful and substantial quote, let it guide you when writing the topic sentence. Remember that the topic sentence works as the thesis of a CHUNK paragraph. The topic sentence needs to tell the reader what the subject/topic of the paragraph will be. 5. STEP BY STEP Commentary/Analysis The commentary/analysis should simply and concisely explain the quote. Discuss the quote within the context of that particular moment. Don’t write about what will happen far into the future or what has already happened. Focus on explaining and analyzing only what the quote is trying to convey. 6. STEP BY STEP Concluding Sentence The concluding sentence should not be a summary of the paragraph. Instead it should explain the result, consequences, or what occurred immediately after the scene described in the quote. 7. CHUNK PARAGRAPH EXAMPLE 8. Auggie believes that true friendship is unconditional and he must help Jack even if it might put him in danger. In the woods, while at camp, Auggie thinks, “This was the point where I knew I should runaway as fast as I could but Jack was still on the ground and I wasn’t about to leave him” (266). Jack and Auggie are being bullied by seventh graders after theY see Auggie and begin to make fun of him. Jack is on the ground because he defended Auggie against the seventh graders; in return they push Jack. As a result, Jack tells Auggie to run so that he doesn’t get hurt, but Auggie, although he might be risking his health, he believes that he must stay and help defend Jack. 9. CHUNK PARAGRAPH EXAMPLE The mask that Auggie wears is a symbol of his need to be normal. On the morning of Halloween, Auggie thinks, “I get to wear a mask, I get to go around like every other kid, and nobody thinks that I look weird. Nobody takes a second look and nobody notices me” (73) Auggie has a facial deformity and longs to be accepted by his peers. Wearing the mask allows Auggie to be that normal kid at least one day of the year. As a result, no one stares at Auggie and other students high-five him in approval of him because he is hiding behind his Halloween mask.
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