I could have written a book about love following exactly the same model as I have here for disgust. I’d have proposed its central adaptive function, traced the components of the behavioral tasks and their evolution in animals in different ecological niches. I’d have proposed how to disassociate distinct subcomponents of love according to the different types of problems that it typically meets, or the kinds of sacrificial behavior it motivates.11 I could have described love as an adaptive system, varying by individuals due to innate factors and/or experience over the lifetime. I could have shown how love aids learning, with the reward system using signs of the happy other to tune the love behaviors of individuals. I could have looked at how love affects culture and how culture affects love. I could have investigated the pathologies of the love system, perhaps in religious fervor and stalking behavior. This alternative book about love would also have drawn conclusions about why love matters—lessons that could be important for human flourishing.
We can, and we need to, unpick love, and indeed all of the emotions, from the theoretical perspective of their ultimate evolutionary function, as we have done for disgust, if we are going to study emotion scientifically.
In the end, instead of writing about love, I chose to write a book about disgust. Intellectual curiosity was one reason; the other was my concern that this dark side of our emotional nature is neglected at our peril. As the instinct that protects us from microbes, it helps keep us healthy. As a factor behind manners, it helps us to be social, but also causes exclusion, stigma, and misery when it misfires. And as a component in our moral choices, disgust can hinder or help us build flourishing societies.
Writing this book has convinced me that figuring out how human behavior works, whether through the lens of disgust or of other motives such as love, could hardly be a more important task for us as a self-aware species. And I’m delighted to be living in a time when I can expect to see the next generation of scientists finally get to grips with what drives human behavior. One thing is certain: the story is only just beginning!
ACKNOWLEDGMENTS
This book benefited immeasurably from the wisdom of my collaborator and partner Robert Aunger. I would also like to thank Justin Aunger, Mícheál de Barra, Adam Biran, Sandy Cairncross, Sylvia Cremer, Dan Fessler, Joann Hoy, Jamie Large, Myriam Sidibé, Panos Tsagamoulis, Josh Tybur, and an anonymous reviewer for their advice. Christie Henry and Latha Menon ably and enthusiastically edited and steered the book to publication. My kids, Naïma and Abidine Sakandé, were an inspiration.
Chapter 6 is based on work previously published as V. Curtis, “Why Disgust Matters,” Philosophical Transactions of the Royal Society, London B Biol Sci 2011; 366 (1583):3478–90.
APPENDIX: THE LONDON DISGUST SCALE
Results
Food/Animal Disgust 3.5
1. You crack open a boiled egg only to find a partially developed chick fetus inside. 4.2
2. You are served a dish made of cow’s tongue and cheek. 3.8
3. Finding a dead mouse in the corner of your kitchen. 3.5
4. You pour lumpy stale milk on your cereal. 3.7
5. You see a cockroach run across your path. 2.5
Sex Disgust 2.9
1. A street prostitute offers you sex for money. 2.2
2. Shortly after meeting someone, you take them back to your house and have sex. 2.7
3. Hearing about a woman who had sex with seven people in one day. 3.1
4. You discover that your romantic partner once paid for sexual intercourse. 3.6
5. A friend admits to attempting sexual intercourse with a piece of fruit. 2.7
Lesion Disgust 2.7
1. You see a nurse dressing an infected wound; under the yellow bandages there is a weeping sore. 2.6
2. Your friend shows you a big oozing lesion on his foot. 2.7
3. On a medical TV program, you see some blisters on a male’s genitals. 3.0
4. Someone you work with develops a bad eye infection; the eye is almost fully sealed and weeps constantly. 2.5
5. At a medical history museum, you see a wax model showing the effects of syphilis on the male and female body. 2.7
Atypical Appearance Disgust 1.8
1. Sharing an elevator with a man with a disfigured face. 1.3
2. Shaking hands with a homeless man. 1.4
3. A woman with unkempt hair and disheveled clothes sits beside you on the bus. 2.2
4. In a crowd you notice a man with one empty eye socket. 2.0
5. Seeing an obese woman sunbathe. 1.9
Hygiene Disgust 3.5
1. Seeing some snotty tissues left on the table. 3.1
2. You see someone sneeze phlegm onto their hands. 3.9
3. Watching a woman pick her nose. 3.2
4. On the subway, you are forced to stand close to someone with body odor and greasy hair. 3.7
5. Feeling something sticky on a door handle. 3.5
Fomite Disgust 2.6
1. Without realizing, you use the dog’s brush to brush your own hair. 2.6
2. A stray dog licks you on your face. 3.1
3. You accidentally use someone else’s roll-on deodorant. 2.1
4. A piece of toast drops butter-side down on the kitchen floor. You’re hungry and it looks clean, so you pick it up and eat it anyway. 2.3
5. At a restaurant, you notice you have accidentally been eating with a fork used by the person next to you. 3.0
Total 2.8
N = 550.
NOTES
Preface
1. Four books on disgust appeared in 2011/12: D. Kelly, Yuck! The Nature and Moral Significance of Disgust (Cambridge, MA: MIT Press, 2011); C. McGinn, The Meaning of Disgust (Oxford: Oxford University Press, 2011); R. Herz, That’s Disgusting: Unraveling the Mysteries of Repulsion (New York: W. W. Norton and Company, 2012); C. Korsmeyer, Savoring Disgust: The Foul and the Fair in Aesthetics (Oxford: Oxford University Press, 2011). I disagree with most of the explanations for disgust that are found in these volumes. Other recent surveys of disgust include W. I. Miller, The Anatomy of Disgust (Cambridge, MA: Harvard University Press, 1997); and R. Rawdon Wilson, The Hydra’s Tale: Imagining Disgust (Edmonton: University of Alberta Press, 2002). Two early twentieth-century studies of disgust are worth reading: A. Kolnai, On Disgust, trans. E. Kolnai (1927; Peru, IL: Carus, 2004); and W. Menninghaus, Disgust: The Theory and History of a Strong Sensation (New York; State University of New York Press, 2003).
2. The New York Times piece (“Survival’s Ick Factor,” by J. Gorman) can be found at http://www.nytimes.com/2012/01/24/science/disgusts-evolutionary-role-is-irresistible-to-researchers.html?pagewanted=all.
3. G. C. M. Jansen, A. O. Koloski-Ostrow, and E. M. Moormann, eds., Roman Toilets: Their Archaeology and Cultural History (Leuven, Belgium: Peeters Publishers, 2011); M. Bradley and K. Stow, eds., Rome, Pollution and Propriety: Dirt, Disease and Hygiene in the Eternal City from Antiquity to Modernity (Cambridge: Cambridge University Press, 2012).
4. J. M. Gottman, What Predicts Divorce? The Relationship between Marital Processes and Marital Outcomes (Hillsdale, NJ: Lawrence Erlbaum, 1994).
5. Paul Rozin and Jonathan Haidt did most of the pioneering scientific work on the topic of disgust. Their approach is summarized in P. Rozin, J. Haidt, and C. McCauley, “Disgust,” in Handbook of Emotion, ed. M. Lewis, J. M. Haviland-Jones, and L. F. Barrett (New York: Guilford Press, 2008).
6. M. Douglas, Purity and Danger: An Analysis of Concepts of Pollution and Taboo (London: Routledge and Kegan Paul, 1966).
7. Robert Rawdon Wilson calls his book The Hydra’s Tale in honor of disgust’s many heads.
8. From the title of Richard Dawkins’s book Unweaving the Rainbow (London: Allen Lane, 1998).
9. For brevity throughout this book, I use the words parasite and pathogen interchangeably. Biologically speaking, pathogens are a subcategory of parasites; however, parasitologists and epidemiologists have more specific meanings for the terms, parasites tending to be
multicellular organisms and pathogens being microbial and viral.
Chapter One
1. D. Heymann, ed., Control of Communicable Diseases Manual, 19th ed. (Washington, DC: American Public Health Association, 2008).
2. L. Liu et al., “Global, Regional, and National Causes of Child Mortality: An Updated Systematic Analysis for 2010 with Time Trends since 2000,” Lancet 379 (2012).
3. A few authors prior to this had suggested that disease and disgust might be linked; for example, Randy Nesse and George C. Williams suggested that instinctive disgust keeps us from feces, vomit, and sources of contagion, and Steven Pinker called disgust “instinctive microbiology.” R. M. Nesse and G. C. Williams, Evolution and Healing (London: Weidenfeld and Nicolson, 1995); S. Pinker, How the Mind Works (London: Penguin, 1998). Earlier than this, anthropologists such as Émile Durkheim and Marvin Harris attempted to explain the purification rituals and food taboos of different cultures as a sort of primitive hygiene. Mary Douglas labeled such thinking as “naïve medical materialism.” Douglas, Purity and Danger. We were, however, first to provide a detailed account of the relationship between disease and disgust elicitors; see V. Curtis and A. Bi-ran, “Dirt, Disgust, and Disease: Is Hygiene in Our Genes?” Perspectives in Biology and Medicine 44, no. 1 (2001); V. Curtis, M. de Barra, and R. Aunger, “Disgust as an Adaptive System for Disease Avoidance Behaviour,” Philosophical Transactions of the Royal Society B 366 (2011).
4. These data, collected by master’s student Panos Psagamoulis from a free listing of disgust items by passengers at Athens international airport, were published in Curtis and Biran, “Dirt, Disgust, and Disease.”
5. It might be argued that the diseases I’ve cited are those that are found in medical textbooks today and are not the diseases that were around at the time that we evolved these aversions. Our modern human parasites are special in the sense that they have evolved in response to our recent shift to group living and animal husbandry, and they have evolved on a different trajectory in the time since we split from the great apes. But actually this is nothing special; evolutionary splits and changes in ecological circumstances are the story of all animals. All animals still have to deal with essentially the same pattern of viral, bacterial, fungal, protozoal, helminthic, and arthropodal parasites. Primates suffer from salmonellosis, giardiasis, candidiasis, malaria, schistosomiasis, hepatitis, TB, leprosy, Ebola, polio, ascariasis, pinworm, and amebiasis caused by the same, or cousin, agents to those causing human disease. Relations are close enough across primates for parasite cross-infection in both directions to have become a major issue for primatologists.
6. The use of words such as instinctual and innate have a long and contested history. See M. Mameli and P. Bateson, “Innateness and the Sciences,” Biology and Philosophy 21, no. 2 (2006), for a discussion. Previously I have suggested that disgust is an instinct that has to be learned—paraphrasing Mineka and Cook on the fear of snakes: S. Mineka and M. Cook, “Mechanisms Involved in the Observational Conditioning of Fear,” Journal of Experimental Psychology, General 122 (1993). A more precise usage here might be “organized in advance of experience,” a term coined by G. F. Marcus, The Birth of the Mind: How a Tiny Number of Genes Creates the Complexities of Human Thought (New York: Basic Books, 2004), which I took from J. Haidt, The Righteous Mind: Why Good People Are Divided by Politics and Religion (London: Penguin Books, 2010).
7. M. Fumagalli et al., “Signatures of Environmental Genetic Adaptation Pinpoint Pathogens as the Main Selective Pressure through Human Evolution,” PLOS Genetics 7, no. 11 (2011).
8. Throughout this book I’ve referred to disgust and other motives as if they have agency, as if they “pull our strings” or tell us what to do. Some might argue that this is poetic license. Technically what is happening is that some aspect of the neurology of the brain causes an animal to bias its choices toward one course of action rather than another. These produce statistically discernible regularities in response to similar classes of stimuli in the environment, all else being equal. There is, of course, no actual voice or direct impulsion.
9. Curtis and Biran, “Dirt, Disgust, and Disease.”
10. J. Haidt, C. McCauley, and P. Rozin, “Individual Differences in Sensitivity to Disgust: A Scale Sampling Seven Domains of Disgust Elicitors,” Personality and Individual Differences 16, no. 5 (1994); P. Rozin et al., “Individual Differences in Disgust Sensitivity: Comparisons and Evaluations of Paper-and-Pencil versus Behavioral Measures,” Journal of Research in Personality 33, no. 3 (1999).
11. A full account of Mícheál’s research that resulted in our new disgust scale can be found in M. de Barra, “Attraction and Aversion: Pathogen Avoidance Strategies in the UK and Bangladesh” (PhD diss., London School of Hygiene and Tropical Medicine, 2011). Factor analysis is one of the best tools we have in psychology for pulling apart the mental mechanisms in the brain; however, the data it produces can leave room for interpretation. It looked as though, with a larger sample, the food and animal disgust might have broken into two factors, and the same might have happened for disease-risk sex, incest, and rule-breaking sex. This scale, which we’ve called the London Disgust Scale, can be found in the appendix.
12. Again see Mameli and Bateson, “Innateness and the Sciences.”
13. E. Cashdan, “Adaptiveness of Food Learning and Food Aversions in Children,” Social Science Information 37, no. 4 (1998).
14. R. M. Nesse, “Natural Selection and the Regulation of Defenses: A Signal Detection Analysis of the Smoke Detector Principle,” Evolution and Human Behavior 26 (2005).
15. The field of disgust is still rife with such controversies.
16. See the excellent book on the history of controversies over sociobiology and evolutionary psychology by Kevin Laland and Gillian Brown, Sense and Nonsense: Evolutionary Perspectives on Human Behaviour (Oxford: Oxford University Press, 2011). See also U. C. O. Segerstråle, Defenders of the Truth: The Sociobiology Debate (Oxford: Oxford University Press, 2001).
17. C. Darwin, The Expression of the Emotions in Man and Animals (1872; reprint, Chicago: University of Chicago Press, 1965).
18. Ibid.
19. This idea is was entrenched by the man who did the most for the science of disgust, Paul Rozin, who came to the topic via studies of taste aversions in rats. It is repeated in almost every work on disgust, for example in W. I. Miller, Anatomy of Disgust; S. B. Miller, Disgust: The Gatekeeper Emotion (Hillsdale, NJ: Analytic Press, 2004); Herz, That’s Disgusting; Kelly, Yuck!
20. H. Spencer, The Data of Ethics, vol. 9 (London: Williams and Norgate, 1887), 206.
21. Mary Douglas was familiar with the Hindu purity-based caste system, which must have inspired her writing about the symbolic nature of dirt, though the book mostly concerns other cultures. Douglas invited me to speak at a seminar in honor of her eightieth birthday at University College, London, where she held to her view that disgusting matter was a product of cultural forces, while I said that it was a product of our biology, much older than society. I didn’t win the debate with that audience of her devotees, but it was inspiring to see this sparkling figure wrestling with new ideas in her ninetieth decade.
22. S. Freud, J. Strachey, and A. Richards, On Sexuality: Three Essays on the Theory of Sexuality and Other Works (1905) (London: Penguin Books, 1977).
23. Kolnai, On Disgust.
24. A. Angyal, “Disgust and Related Aversions,” Journal of Abnormal and Social Psychology 36 (1941).
25. Rozin’s status and that of his work are pointed out in J. M. Tybur et al., “Disgust: Evolved Function and Structure,” Psychological Review 120, no. 1 (2013).
26. Rozin, Haidt, and McCauley, “Disgust.”
27. Rawdon Wilson, Hydra’s Tale.
28. Curtis, de Barra, and Aunger, “Disgust as an Adaptive System.”
Chapter Two
1. A. M. Kuris et al., “Ecosystem Energetic Implications of Parasite and Free-Living Biomass in Three Estuaries,” Nature 454, no. 7203 (2008); C. Zi
mmer, Parasite Rex (New York: Touchstone, 2000).
2. We come equipped with most of the abilities of the animals in our ancestral phylogeny, and our brains betray this. See, for example, G. F. Streidter, Principles of Brain Evolution (Sunderland, MA: Sinauer Associates, 2005); J. Panksepp, Affective Neuroscience (Oxford: Oxford University Press, 1998). Of course, we are generalists with abilities to learn from our environments in ways that way that outstrip our animal ancestors. But this does not mean that we have lost the ancient ancestral reflexive and motivational response systems: S. J. Shettleworth, “Modularity, Comparative Cognition and Human Uniqueness,” Philosophical Transactions of the Royal Society B 367, no. 1603 (2012); R. Aunger and V. Curtis, “Kinds of Behaviour,” Biology and Philosophy 23, no. 3 (2008).
3. Parasite-avoidance behavior in animals hasn’t been a major focus of zoological investigation. See, for example, B. L. Hart, “Behavioural Defences in Animals against Pathogens and Parasites: Parallels with the Pillars of Medicine in Humans,” Philosophical Transactions of the Royal Society B 366, no. 1583 (2011); S. Cremer, S. A. O. Armitage, and P. Schmid-Hempel, “Social Immunity,” Current Biology 17, no. 16 (2007).
4. M. Kavaliers and D. D. Colwell, “Discrimination by Female Mice between the Odours of Parasitized and Non-Parasitized Males,” Proceedings of the Royal Society of London B 261 (1995).
5. M. F. Spurier, M. S. Boyce, and B. F. J. Manly, “Effect of Parasites on Mate Choice of Captive Sage Grouse,” in Bird-Parasite Interactions: Ecology, Evolution, and Behaviour, ed. J. E. Loye and M. Zuk (Oxford: Oxford University Press, 1991).
6. C. Loehle, “Social Barriers to Pathogen Transmission in Wild Animal Populations,” Ecology 76 (1995).
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