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psychologist vol 22 no 11

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Parasites, minds and cultures Could the most human of qualities owe their existence to tiny, mindless organisms? Justin Park and Mark Schaller investigate, as part of a special issue marking this year’s Darwin anniversaries

Incorporating Psychologist Appointments £5 or free to members of The British Psychological Society

forum 906 news 912 careers 970 looking back 988

developing human brain functions 924 beyond ‘just-so stories’ 930 class through the evolutionary lens 934 the peacock’s tail of altruism 938

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The British Psychological Society Contact The British Psychological Society St Andrews House 48 Princess Road East Leicester LE1 7DR Tel 0116 254 9568 Fax 0116 227 1314 Society website The Psychologist e-mail General Society e-mail Advertising Reach 50,000 psychologists at very reasonable rates. For rates and deadlines, e-mail or tel 0116 252 9552 For job advertising, in print or online at, e-mail tel 0116 252 9550 October 2009 issue 47,289 dispatched Printed by Warners Midlands plc, Bourne, on 100 per cent recycled paper Please re-use or recycle See the online archive at ISSN 0952-8229 © Copyright for all published material is held by The British Psychological Society, unless specifically stated otherwise. Authors, illustrators and photographers may use their own material elsewhere after publication without permission. The Society asks that the following note be included in any such use: ‘First published in The Psychologist, vol. no. and date. Published by The British Psychological Society – see’ As the Society is a party to the Copyright Licensing Agency (CLA) agreement, articles in The Psychologist may be photocopied by licensed institutional libraries for academic/teaching purposes. No permission is required. Permission is required and a reasonable fee charged for commercial use of articles by a third party. Please apply to the Society in writing. The publishers have endeavoured to trace the copyright holders of all illustrations in this publication. If we have unwittingly infringed copyright, we will be pleased, on being satisfied as to the owner’s title, to pay an appropriate fee.

Welcome to The Psychologist, the monthly publication of The British Psychological Society. It provides a forum for communication, discussion and controversy among all members of the Society, and aims to fulfil the main object of the Royal Charter, ‘to promote the advancement and diffusion of a knowledge of psychology pure and applied’. It is supported by, where you can view this month’s issue, search the archive, listen, debate, contribute, subscribe, advertise, and more.

Welcome to a small selection of material from the forthcoming issue of The Psychologist, the monthly publication of the British Psychological Society. To purchase a PDF of this issue, see If you are studying for a psychology qualification or have gained one in the past, you may be eligible to join the Society. See for more details. For more on brain and behaviour, see and the Society’s free Research Digest service at Editor Jon Sutton Assistant Editor Peter Dillon-Hooper Production Mike Thompson Staff journalist Christian Jarrett Advertising Sarah Stainton Kirsty Wright Editorial Assistant Debbie James

Associate Editors Articles Vaughan Bell, Kate Cavanagh, Marc Jones, Rebecca Knibb, Charlie Lewis, Amina Memon, Wendy Morgan, Tom Stafford, Miles Thomas, Monica Whitty, Barry Winter

Conferences Sandie Cleland Sarah Haywood International Nigel Foreman, Asifa Majid Interviews Nigel Hunt, Lance Workman History of Psychology Julie Perks

The Psychologist Policy Committee Paul Redford (Chair), Nik Chmiel, Olivia Craig, Helen Galliard, Jeremy Horwood, David Lavallee, Catherine Loveday, Stephen McGlynn, Sheelagh Strawbridge, Henck van Bilsen, Peter Wright, and Associate Editors

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forum 906 taking A-level psychology seriously; in defence of empiricism; and much more


news, conference and media 912 post-antibiotic apocalypse; walking in circles; reports from the British Science Festival and a Martin Seligman lecture; and Kisane Prutton on the benefits of engaging with a changing media

150 years ago this month, Charles Darwin’s On the Origin of Species changed the world. This issue celebrates that anniversary with a range of contributions from modern psychologists taking Darwin’s theories into new and intriguing areas. Take parasites. To Darwin, they were fascinating examples of adaptation. But recent research (see p.942) has uncovered a set of psychological adaptations that serve as a first line of behavioural defence against contact with parasites. This ‘behavioural immune system’ has provocative implications for many different kinds of phenomena that are of interest to psychologists. Another anniversary we celebrate this month is the 150th e-mail issue of the Research Digest. See p.922 for some top psychologists revealing ‘one nagging thing’ they still don’t understand about themselves. Dr Jon Sutton (Managing Editor)


Developing human brain functions Mark Johnson describes the emerging field of developmental cognitive neuroscience


Beyond ‘just-so stories’ Lisa DeBruine on how evolutionary theory leads to novel predictions


Social class through the evolutionary lens Daniel Nettle takes a look


The peacock’s tail of altruism Wendy Iredale and Mark Van Vugt on the Darwinian psychology of helping


methods evaluating explanatory theories: Brian Haig advocates ‘inference to the best explanation’, a method used by Darwin


book reviews mental toughness; consciousness; the compassionate mind; and more


society President’s column; Presidents’ Award; accreditation; journals; and more


Blogging on brain and behaviour JOHN STURROCK/REPORTDIGITAL.CO.UK


970 careers Darwin in the workplace; the Society’s Undergraduate Research Assistantship scheme; the latest jobs, and how to advertise looking back on the origins of human nature: Chris Lerwill digs into Darwin’s archives


one on one …with David Buss


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One nagging thing… Top psychologists help us to celebrate the 150th Research Digest e-mail


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One nagging thing… Top psychologists help us to celebrate the 150th e-mail issue of the Society’s Research Digest

Still fooled

he British Psychological Society’s Research Digest was launched in 2003, as a fortnightly e-mail service. Over the years it has added (now ranked as the 16th most influential science blog in the world by Wikio), a Facebook site and a Twitter feed. It is read and appreciated by many thousands of psychologists and others across the world. To mark the 150th e-mail edition, its editor, Dr Christian Jarrett, asked some of the world’s leading psychologists to describe in 150 words ‘one nagging thing you still don’t understand about yourself’. Their answers featured on the blog, The Independent newspaper and websites internationally, but for those of you who missed it we publish a selection here.

One nagging thing I don’t understand about myself is why I’m still fooled by incidental feelings. Some 25 years ago Jerry Clore and I studied how gloomy weather makes one’s whole life look bad – unless one becomes aware of the weather and attributes one’s gloomy mood to the gloomy sky, which eliminates the influence. You’d think I learned that lesson and now know how to deal with gloomy skies. I don’t, they still get me. The same is true for other subjective experiences, like the processing fluency resulting from print fonts – I still fall prey to their influence. Why does insight into how such influences work not help us notice them when they occur? What makes the immediate experience so powerful that I fail to apply my own theorising until some blogger asks a question that brings it to mind? Norbert Schwarz, Professor of Psychology at the University of Michigan


Puzzling love for our children I’ve had three of my own children and spent my professional life thinking about children. And yet I still find my relation to my children deeply puzzling. Our love for children is so unlike any other human emotion. I fell in love with my babies so quickly and profoundly, almost completely independently of their particular qualities. And yet 20 years later I was (more or less) happy to see them go – I had to be happy to see them go. We are totally devoted to them when they are little and yet the most we can expect in return when they grow up is that they regard us with bemused and tolerant affection. We are ambitious

Nagging need for more? Visit to read more contributions, from Susan Blackmore, Robert Cialdini, Sue Gardner, Jerome Kagan, David Lavallee, Robert Plomin, Mike Posner, Steve Rose, Paul Rozin, Martin Seligman, Robert Sternberg and Richard Wiseman. While you’re there, sign up for the e-mail Digest and join us on the journey of the next 150 issues.


pandemonium, our destiny (we hope) is Caledonian. Who do we want to be? What will others let us be? And does it count one jot to anyone but me? No wonder I study identity. Steve Reicher is Professor of Psychology at the University of St Andrews

for them, we want them to thrive so badly. And yet we know that we have to grant them the autonomy to make their own mistakes. In no other human relation do we work so hard to accomplish such an ill-defined goal, which is precisely to create a being who will have goals that are not like ours. Alison Gopnik, Professor of Psychology at the University of California at Berkeley

Satiators and addicts I’ve been told that there are two kinds of people in the world: Satiators and Addicts. Satiators get their fill of something, and that’s enough for the rest of their lives. For example, I’m that way about beaches: I grew up a 10-minute walk from the Pacific Ocean, and went to the beach practically every day during my adolescence. But enough was enough, and I now don’t care whether I ever see a beach again. In contrast, Addicts get hooked, and never get enough of something. I’ve obsessed about the same narrow research topic for over 35 years, and the end is not in sight. Why am I a Satiator in some cases, and an Addict in others? Stephen Kosslyn, Dean of Social Science and Professor of Psychology at Harvard

Who am I? Who am I? I am a jew, but I am no believer and I do not believe that Israel speaks for me. I can’t be sure what it means to be a jew. Yet I am sure that others are sure. And I know that jewishness matters. I know that millions were slaughtered for being jewish. I know that millions have been displaced by jews for not being jewish. What is being jewish to my world and to me? Who are we? Who am I? I was born in England of family who fled from Germany and Poland. I was raised in England by parents who moved abroad for work. I live in Scotland with a wife born in Yorkshire of a father born in Pakistan and with a son born in Scotland. Our history is

Optimism When my dear friend and colleague Roger Brown was alive he used to say that to him, I define the edge of the optimism continuum. I think my outlook explains my choice of research topics. Instead of describing what is, most of my work is aimed at exploring what might be. In my most recent book I discuss extending what we take as limits to our physical health and well-being. I don’t understand why I’m so confident that we’ve just scratched the surface of what our consciousness is capable of, but every year and every experiment I do makes me more certain that the future will only vaguely resemble the past in this regard. I don’t know how I came to these views, or whether in the long run people like me will ‘win or lose’ to the cynics. One thing I do know, however, is while the future unfolds people like me are having a better time as we consider all sorts of possibility. So, I remain optimistic about being optimistic. Ellen Langer, Professor of Psychology at Harvard University

Death and forgiveness In my recent conversations with the Dalai Lama we disagreed about two matters. One

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was fear of death, which I claim not to feel and he claims everyone has. The evidence is in his favour since all religions promise life of some kind after death, and they would not do so if people didn’t need it. I fear a painful death, but not death itself. Can’t comprehend why people do; which doesn’t mean I don’t wish to continue living, but as time progresses and body parts and the mind wears out I expect death will be welcome. Our other disagreement was about forgiveness. I believe there are unforgiveable actions – child abuse, rape, holocausts, torture are examples. The Dalai Lama says he forgives but does not forget. In my view, since he believes such people will be reincarnated in an undesirable form, he doesn’t need to forgive them. Paul Ekman, Manager of the Paul Ekman Group, LLC (PEG)

Beauty What is this thing I call beauty? Not ‘art’ as a social phenomenon based on status or display, or beautiful faces seen merely as biological fitness markers. Rather, the sheer, drawing-in-of-breath beauty of a Handel aria, a Rothko painting, T.S. Eliot’s poems, or those everyday moments of sun shining through wet autumn leaves, or even a PowerPoint layout seeming just right. Content itself doesn’t matter – Cézanne’s paintings of apples are not beautiful because one likes apples, and there are beautiful photographs of horrible things. Somewhere there must be something formal, structural, compositional, involving the arrangement of light and shade, of sounds, of words best ordered to say old ideas in new ways. When I see beauty, I know it; and others must also see it, or they wouldn’t make the paintings I like or have them hung in galleries. But why then doesn’t everyone see it in the same way? Chris McManus Professor of Psychology and Medical Education at UCL

Nightmares I don’t understand why I have nightmares almost every night. Nightmares of frustration. Obstacles in my way that keep me from catching an airplane trip on time. Obstacles that keep me from getting where I’m supposed to be. I wake up almost every morning with a sense of relief – ‘Thank goodness it was just a dream.’ None of my colleagues seem to spend their nights this way. What possible reason is there for this mental behaviour, night after night, that is clearly so uncomfortable? One colleague, a developmental psychologist, said: ‘That’s it – the happy relief you feel at the end. There’s your reinforcement.’ And thus she took away my one idea, by explaining it. It is now one nagging thing that I only partly understand. Or do I? Elizabeth Loftus, Professor of Psychology at the University of California, Irvine

Lost opportunities Why didn’t I ask my grandparents before they died, more about their childhoods? ‘Grandpa, what was it like being born in 1900 into a world where man couldn’t fly and an abacus was the closest thing to a computer?’ ‘Grandpa, did it hurt when grandma burnt the leeches off your back on your return from the trenches, as you sat in the tin bath in front of the fire?’ ‘Nana, did you enjoy being one of the

first families in Sunderland to own an “automobile” and having to eat “below stairs” with the cooks and the maids?’ ‘Nana, how did you cope as the youngest of 12 in a poor, Derbyshire, farming family, gaining a scholarship to grammar school, but being forced to go away into service at 13 to become a scullery maid?’

live? But here’s a nagging thought: might those two preoccupying questions turn out to be one and the same, like the evening star and the morning star? Paul Broks, a clinical neuropsychologist at the University of Plymouth

Marilyn Davidson, Professor of Work Psychology at Manchester Business School

Why do I often succumb to welldocumented psychological biases, even though I’m acutely aware of these biases? One example is my failure at affective forecasting, such as believing that I will be happy for a long time after some accomplishment (e.g. publishing a new book), when in fact the happiness dissipates more quickly than anticipated. Another is succumbing to the male sexual overperception bias, misperceiving a woman’s friendliness as sexual interest. A third is undue optimism about how quickly I can complete work projects, despite many years of experience in underestimating the time actually required. One would think that explicit knowledge of these well-documented psychological biases and years of experience with them would allow a person to cognitively override the biases. But they don’t. David Buss, Professor of Psychology at the University of Texas

What should I do? There’s plenty I don’t understand about myself, but nothing nags. Paradoxically, the deeper I got into neuropsychology the less interested I became in the details of my own inner workings. I’m not sure why. It certainly is not because I arrived at any great insight or understanding. I still experience the almost visceral sense of puzzlement over matters of brain, mind and selfhood that first drew me to the field. What happened, I think, was a shift – let’s imagine a neural switch somewhere in the frontolimbic circuitry – from one preoccupying question, What am I?, to another, What should I do? It left me less inclined to bother about selfunderstanding than to consider the value of things, moral and aesthetic. How best to

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Overcoming irrationality



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Parasites, minds and cultures Could the most human of qualities owe their existence to tiny, mindless organisms? Justin H. Park and Mark Schaller investigate he littlest of things can have huge evolutionary significance. Before publishing The Origin of Species Charles Darwin spent years studying barnacles. If Darwin were alive today, we suspect that he would be mightily impressed by what we now know about the evolutionary impact of much smaller and more ancient things: viruses, bacteria, protozoa and intestinal worms that parasitise bigger organisms. Where there is life, there are parasites – in immeasurable abundance. These parasites can seriously impair the health and reproductive fitness of the organisms that they infect. All living animals – including humans – are around today because their ancestors evolved ways to elude parasites, generation after generation. This evolutionary process has left huge footprints that researchers are just starting to discern (Ridley, 1993; Zimmer, 2000; Zuk, 2007).


What might be some evolutionarily adaptive reactions to people with infectious diseases?


How has our long history of living with parasites shaped how we think, feel and behave?

Schaller, M. & Duncan, L.A. (2007). The behavioral immune system: Its evolution and social psychological implications. In J.P. Forgas, M.G. Haselton & W. von Hippel (Eds.) Evolution and the social mind: Evolutionary psychology and social cognition (pp.293–307). New York: Psychology Press.



Parasites have had profound effects on human evolution. Recent research implicates the existence of a set of psychological adaptations that serve as a first line of behavioural defence against contact with parasites – the ‘behavioural immune system’. The ordinary operation of the behavioural immune system has provocative implications for many different kinds of phenomena that are of interest to psychologists – including stigmatisation and prejudice, physical attractiveness and mating behaviour, and the origins of cultural diversity.

Curtis, V., Aunger, R. & Rabie, T. (2004). Evidence that disgust evolved to protect from risk of disease. Proceedings of the Royal Society of London B, 271, S131–S133. Faulkner, J., Schaller, M., Park, J.H. & Duncan, L.A. (2004). Evolved diseaseavoidance mechanisms and contemporary xenophobic attitudes. Group Processes and Intergroup Relations, 7, 333–353.


The evolution of anti-parasite defence systems Parasites don’t look or act like lions or tigers, so people don’t usually think of them as predators. But, in a sense, they are. Parasites attach themselves to the body of a host and exploit bodily resources in order to reproduce. In doing so, they can harm that host, sometimes lethally. As a consequence, host species have evolved elaborate anti-parasite defence systems. We are all familiar with one of these defence systems: the immune system. The immune system is an amazingly

Fincher, C.L., Thornhill, R., Murray, D.R. & Schaller, M. (2008). Pathogen prevalence predicts human crosscultural variability in individualism/ collectivism. Proceedings of the Royal Society B, 275, 1279–1285. Fink, B. & Penton-Voak, I. (2002). Evolutionary psychology of facial attractiveness. Current Directions in Psychological Science, 11, 154–158. Gangestad, S.W., Haselton, M.G. & Buss,

sophisticated suite of adaptations, designed by natural selection to detect parasites that intrude on our bodily tissues and – once those parasites are detected – to mobilise physiological means of repelling, killing or neutralising them. While an immune system has undeniable benefits, it has undeniable drawbacks. Mounting an immune response consumes considerable metabolic resources, which may result in temporary debilitation (e.g. fatigue, exhaustion) while the parasitic infection is being fought. Specific kinds of immune responses (e.g. fever) can be further debilitating. Most importantly, the immune system is incapable of the simplest form of defence: preventing parasites from coming into contact with the body in the first place. It has thus been suggested that animals evolved an additional system of defence that enables them to physically avoid germy things and other infected hosts. This system is designed to employ perceptual cues (appearance, odour, etc.) to detect the presence of infectious parasites in other things – including other individuals. In some animals – including humans – the detection of such cues may trigger aversive emotional and cognitive responses that motivate behavioural avoidance. This behavioural mechanism offers a first line of defence against diseasecausing parasites and hence has been called the ‘behavioural immune system’ (Schaller & Duncan, 2007).

Psychological implications of the behavioural immune system Lots of animal species show evidence of perceptual sensitivity to cues of parasitic infection in other members of their species and of consequent behavioural avoidance (Goodall, 1986; Kavaliers & Colwell, 1995; Kiesecker et al., 1999). Humans are no exception. Recently, there has emerged a body of research exploring the implications of the behavioural immune system for human emotion, cognition and behaviour. For instance, there is evidence suggesting that the

D.M. (2006). Evolutionary foundations of cultural variation: Evoked culture and mate preferences. Psychological Inquiry, 17, 75–95. Goodall, J. (1986). Social rejection, exclusion, and shunning among the Gombe chimpanzees. Ethology and Sociobiology, 7, 227–236. Kavaliers, M. & Colwell, D.D. (1995). Discrimination by female mice between the odours of parasitized

and non-parasitized males. Proceedings of the Royal Society of London B, 261, 31–35. Kiesecker, J.M., Skelly, D.K., Beard, K.H. & Preisser, E. (1999). Behavioral reduction of infection risk. Proceedings of the National Academy of Sciences, 96, 9165–9168. Lafferty, K.D. (2006). Can the common brain parasite, Toxoplasma gondii, influence human culture?

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emotion of disgust evolved to serve as an affective signal of parasite infection (Curtis et al., 2004; Oaten et al., 2009). This line of evidence not only has implications for psychologists’ understanding and measurement of disgust, but also may help to explain why feelings of disgust influence moral judgements and interpersonal relations (e.g. Tybur et al., 2009). Additional implications are emerging as well. In the sections that follow, we briefly review three particularly intriguing sets of findings that illustrate the wideranging psychological implications of the behavioural immune system. One set of findings pertains to the psychology of stigmatisation and prejudice; a second set pertains to the psychology of physical attractiveness and mating behaviour; and a third set bears on the origins of culture and cross-cultural differences.

(erroneously inferring the absence of parasites where, in fact, they exist), and vice versa. How is this signal-detection problem resolved? An answer is provided by the ‘smoke detector principle’ (Nesse, 2005). A smoke detector is typically calibrated to be supersensitive to anything that superficially resembles smoke, in order to minimise the likelihood of failing to register the presence of a house fire (a very costly false-negative error). The inevitable consequence – which people happily tolerate – is lots of (relatively less costly) false-positive errors: the smoke detector may sound its alarm anytime someone is harmlessly braising a steak or boiling a pot of pasta. A similar set of functional priorities applies to the behavioural immune system. In order to avoid the highly costly consequences that may follow from contact with parasites (e.g. illness, death), the system is calibrated to be Stigmatisation and prejudice supersensitive to superficial cues (e.g. Because most parasites are virtually a wide range of morphological or invisible, people behavioural anomalies) must rely on connoting the possible superficial cues presence of parasites. (e.g. anomalous The result is that the physical features) behavioural immune to detect their system may sound its presence. Because alarm (and trigger aversive cues are affective, cognitive and imperfectly behavioural responses) correlated with whenever a person parasitic infection, perceives someone else there emerges a whose superficial physical signal-detection appearance or behaviour problem in which deviates from whatever errors are prototype people perceive inevitable. Any to be ‘normal’. attempt to limit There is another the number of important consideration ‘false-positive’ to keep in mind. Just as the Historically, people suffering from errors activation of the ‘real’ diseases such as leprosy have (erroneously immune system has costs, been highly stigmatised inferring the the activation of the presence of behavioural immune system parasites where has costs as well (e.g. there are none) inevitably leads to consumption of metabolic resources). an increase in ‘false-negative’ errors Therefore, the behavioural immune system

Proceedings of the Royal Society B, 273, 2749–2755. Navarrete, C.D., Fessler, D.M.T. & Eng, S.J. (2007). Elevated ethnocentrism in the first trimester of pregnancy. Evolution and Human Behavior, 28, 60–65. Nesse, R.M. (2005). Natural selection and the regulation of defenses: A signal detection analysis of the smoke detector principle. Evolution and

Human Behavior, 26, 88–105. Oaten, M., Stevenson, R.J. & Case, T.I. (2009). Disgust as a diseaseavoidance mechanism. Psychological Bulletin, 135, 303–321. Park, J.H., Faulkner, J. & Schaller, M. (2003). Evolved disease-avoidance processes and contemporary antisocial behavior: Prejudicial attitudes and avoidance of people with physical disabilities. Journal of Nonverbal

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is likely to be especially supersensitive and especially likely to trigger aversive responses when its benefits are especially likely to outweigh its costs; that is, whenever perceivers are, or merely perceive themselves to be, especially vulnerable to the transmission of disease. This line of reasoning has two broad implications for our understanding of stigmatisation and prejudice. First, the ordinary operation of the behavioural immune system may contribute to the stigmatisation of people whose appearance deviates from some subjective sense of normalcy. Second, prejudicial responses to these people are likely to be exaggerated under conditions in which perceivers are (or merely perceive themselves to be) especially vulnerable to parasite transmission. This analysis thus helps us understand why people suffering from some diseases (such as leprosy) have historically been more highly stigmatised than people suffering from other diseases (which may be more virulent and infectious, but are associated with less overt morphological anomalies). More provocatively, this analysis suggests that psychologically similar prejudicial responses may be directed against individuals who aren’t actually suffering from any infectious disease whatsoever – and that these prejudicial responses vary depending on the extent to which perceivers feel vulnerable to parasite transmission. In our own labs, we have conducted studies to assess the extent to which people are especially likely to implicitly associate the concept ‘disease’ (as well as other aversive cognitions) with specific categories of people. One set of studies implicated the behavioural immune system in implicit prejudices directed against people with superficial facial birthmarks and physical disabilities (Park et al., 2003; Schaller & Duncan, 2007). Another set of studies implicated the behavioural immune system in prejudicial responses to obesity (Park et al., 2007). Among other findings, we discovered that when the threat of

Behavior, 27, 65–87. Park, J.H., Schaller, M. & Crandall, C.S. (2007). Pathogen-avoidance mechanisms and the stigmatization of obese people. Evolution and Human Behavior, 28, 410–414. Penton-Voak, I.S., Jacobson, A. & Trivers, R. (2004). Population differences in attractiveness judgements of male and female faces: Comparing British and Jamaican samples. Evolution and

Human Behavior, 25, 355–370. Rhodes, G. (2006). The evolutionary psychology of facial beauty. Annual Review of Psychology, 57, 199–226. Ridley, M. (1993). The Red Queen: Sex and the evolution of human nature. London: Viking. Schaller, M. & Duncan, L.A. (2007). The behavioral immune system: Its evolution and social psychological implications. In J.P. Forgas, M.G.


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parasite transmission is made temporarily salient (e.g. with a brief slideshow depicting germs and their presence all around), people are especially likely to implicitly associate obese individuals with the semantic concept ‘disease’. These findings not only help to illuminate the causes of weight-based prejudice, they also illustrate the point that the behavioural immune system responds not to rational assessments of parasite infection (after all, parasite infection is more likely to lead to weight loss than weight gain), but instead to relatively crude and wide-ranging perceptual cues. Ethnocentrism and xenophobia also appear to be rooted, in part, in the irrational operation of the behavioural immune system. People who feel especially vulnerable to parasite transmission are especially likely to favour contact with familiar rather than foreign peoples (Faulkner et al., 2004). A particularly provocative finding was reported by Navarrete et al. (2007): women in the first term of pregnancy – whose bodies are naturally immunosuppressed – show especially high levels of xenophobia and ethnocentrism.

and ask more specific questions. Exactly which morphological features are considered to be subjectively attractive? And why exactly do those features (rather than others) connote attractiveness? Answers to that first question include bilateral facial symmetry and feature prototypicality (Fink & Penton-Voak, 2002; Rhodes, 2006). People with more symmetrical faces are perceived to be more attractive, as are people whose specific facial features are closer to the population average (e.g. a nose that is neither especially big nor small, but is just right). But why do these particular features inspire subjective appraisals of attractiveness? An abundance of research in the behavioural ecology literature suggests that, for many species, bilateral symmetry and phenotypic prototypicality may be indicators of a healthy immune system (e.g. Thornhill & Møller, 1997). Applied to humans, the implication is that a subjective appraisal of physical attractiveness may serve as a crude indicator of the extent to which another person is resistant to parasitic infection. Additional, somewhat more sophisticated evolutionary logic provides an explanation for why women are attracted to ‘masculine’ facial features (e.g. Physical attractiveness and strong jaws) that are associated with mating behaviour higher levels of testosterone (Zuk, 2007). Thus far, we have emphasised the point We do not have space here to articulate the that the behavioural immune system is evolutionary analysis in detail, so we sensitive to superficial cues connoting simply emphasise the bottom line – these possible parasite infection. This facial features may point has a logical flip side as also serve as well: in the context of close advertisements to “People with more interpersonal relationships, the potential mates, and symmetrical faces are behavioural immune system is what these features perceived to be more likely to be sensitive to advertise is an attractive” superficial cues connoting especially strong resistance to parasite infection. immune system. A body of research suggests Thus, one reason that subjective appraisals of physical why subjective appraisals of physical attractiveness may serve as such a cue. attractiveness are so important in the We all know that physical mating game is that these appraisals are attractiveness is a rather important helpful in identifying mates who are likely determinant of a person’s sexual allure. to be, and to remain, free from parasitic That is hardly news. But the story becomes infections. There is some evidence that a lot more interesting when we dig deeper individuals – especially women – who are

Haselton & W. von Hippel (Eds.) Evolution and the social mind: Evolutionary psychology and social cognition (pp.293–307). New York: Psychology Press. Schaller, M. & Murray, D.R. (2008). Pathogens, personality, and culture: Disease prevalence predicts worldwide variability in sociosexuality, extraversion, and openness to experience. Journal of


Personality and Social Psychology, 95, 212–221. Sherman, P.W. & Billing, J. (1999). Darwinian gastronomy: Why we use spices. BioScience, 49, 453–463. Thornhill, R., Fincher, C.L. & Aran, D. (2009). Parasites, democratization, and the liberalization of values across contemporary countries. Biological Reviews, 84, 113–131. Thornhill, R. & Møller, A.P. (1997).

subjectively perceived to be physically attractive do, in fact, live healthier lives (Weeden & Sabini, 2005). And, among perceivers, it appears that physical attractiveness is easily learned – and then used – as a cue connoting health (Zebrowitz et al., 2003). This line of reasoning has another interesting implication. If physical attractiveness is an indicator of parasite resistance, then people are especially likely to prioritise physical attractiveness under conditions in which there are more parasites around to contend with. This implication has been tested – and supported – by cross-cultural evidence. In places with more parasites, women show a stronger preference for more masculine faces (Penton-Voak et al., 2004). More generally, regional variation in the presence of parasites predicts cross-cultural variation in the value of attractiveness. In places with historically higher levels of disease-causing parasites, people place a higher value on a potential mate’s physical attractiveness (Gangestad et al., 2006).

The origins of cultural variation We have now entered conceptual territory that may surprise some readers who assume that evolution has no bearing on cross-cultural differences. This assumption is wrong. As we emphasised earlier, evolved psychological mechanisms are highly responsive to the contexts in which people find themselves. Just as the ‘real’ immune system is more chronically activated under ecological circumstances that put people into more regular contact with parasites, the behavioural immune system is also likely to be hyperactive under ecological circumstances characterised by a higher prevalence of parasites. Thus, to the extent that specific kinds of cognitions and behaviours put people at risk for parasite infection, those cognitions and behaviours are likely to predictably vary across human populations, depending on the prevalence of parasites in the local ecology. This has profound implications for our

Developmental stability, disease and medicine. Biological Reviews, 72, 497–548. Tybur, J.M., Lieberman, D. & Griskevicius, V. (2009). Microbes, mating, and morality: Individual differences in three functional domains of disgust. Journal of Personality and Social Psychology, 97, 103–122. Webster, J.P. (2007). The effect of

Toxoplasma gondii on animal behavior: Playing cat and mouse. Schizophrenia Bulletin, 33, 752–756. Weeden, J. & Sabini, J. (2005). Physical attractiveness and health in Western societies: A review. Psychological Bulletin, 131, 635–653. Zebrowitz, L.A., Fellous, J.M., Mignault, A. & Andreoletti, C. (2003). Trait impressions as overgeneralized responses to adaptively significant

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Direct behavioural effects of a parasite? The brain parasite Toxoplasma gondii is mostly harmless and surprisingly common, infecting over 50 per cent of people in some countries. It infects other mammals too (e.g. mice, cats, dogs, deer), and in these other animal populations, T. gondii is known to have effects on the behaviour of its hosts (Webster, 2007). Might it have effects on human psychology and behaviour? Maybe. There is some evidence, for instance, that the prevalence of T. gondii infection in human populations is correlated with cross-cultural differences in traits such as neuroticism and uncertainty avoidance (Lafferty, 2006). These results are probably best described as preliminary, but they are provocative. And they highlight yet another way in which human psychology may be shaped by the presence of parasites.


understanding of cross-cultural differences. Consider, for example, differences in sexual behaviour. Sexual contact has obvious benefits for reproductive fitness, but also potential costs, as it puts people at higher risk for parasitic infections. The ratio of benefits to costs varies, depending on the actual prevalence of parasites in the local ecology. This implies predictable cross-cultural differences in the extent to which people are ‘restricted’ rather than ‘unrestricted’ in their sexual behaviour. Indeed, in regions characterised by a higher prevalence of parasites, people (especially women) are more restricted in their sexual behaviour (Schaller & Murray, 2008). The same logic applies to personality traits such as extraversion and openness to experience. These traits may confer specific kinds of benefits (contact with new friends and new technologies). But both are also likely to be associated with a specific kind of cost: greater exposure to parasites. These costs are greater in regions with a high prevalence of parasites. The implication, supported by empirical evidence, is that in regions characterised by a higher prevalence of parasites, people are less extraverted and less open to new ideas (Schaller & Murray, 2008). Many other cultural norms may also serve as buffers against parasite transmission (especially norms pertaining to hygiene and food preparation; e.g. Sherman & Billing, 1999). This has implications for the emergence of broader systems of cultural values, such as those implicated by the individualism– collectivism dimension that is so important to the study of human cultures. Collectivism is defined in part by an emphasis on conformity to existing traditions and norms, whereas individualism is defined in part by a tolerance for deviance. Individualism therefore connotes a greater risk for

parasite transmission. It follows that collectivistic value systems are especially likely to emerge and persist in regions characterised by a high prevalence of parasites, whereas individualistic value systems are most likely to take hold in regions with a relatively low level of parasites. This appears to be the case (Fincher et al., 2008). These and other findings (e.g. Thornhill et al., 2009) suggest that many important cross-cultural differences may owe their existence, in part, to parasites and to the context-contingent responses of the behavioural immune system that evolved in response to parasites.

Envoi We have focused here on how the human mind is adapted to minimise the threat of parasite infection and on the psychological consequences of those adaptations. But this is not the only means through which parasites may influence human psychology. A rather different line of research explores the possibility that some parasites – when they do successfully infect human beings – may directly manipulate human

facial qualities: Evidence from connectionist modeling. Personality and Social Psychology Review, 7, 194–215. Zimmer, C. (2000). Parasite rex: Inside the bizarre world of nature’s most dangerous creatures. New York: Free Press. Zuk, M. (2007). Riddled with life: Friendly worms, ladybug sex, and the parasites that make us who we are. Orlando, FL: Harcourt.

read discuss contribute at

I Justin H. Park

is with the Department of Experimental Psychology, University of Bristol

cognition and behaviour (see box for an example). Scientific progress is often a humbling affair that demands abandonment of cherished ideas: we are not at the centre of the universe, our bodies are not animated by vital essence, our minds are not ethereal spirits. Judging by the tenacious resistance even today, it would seem that Darwin’s insights instigated some of the most profound – and humbling – pieces of scientific work to date. Science marches on, because the price of forfeiting cherished ideas is offset by the benefit of deeper, fuller understanding of the world and our place in it. The recent advances regarding the evolutionary impact of parasites (of which only a sliver was described in this article) may demand further changes in views. Perhaps nothing is as humbling as learning that the most human of qualities – patriotic feelings, appreciation of beauty, sexual passion, cultural diversity – may owe their existence to tiny mindless disease-causing organisms. But by giving parasites their due attention, we are beginning to get a handle on some of the oldest questions and discovering that psychology is even more profound than it first appears.

I Mark Schaller

is with the Department of Psychology, University of British Columbia


The Psychologist November 2009