Thus far, we have emphasised the point that the behavioural immune system is sensitive to superficial cues connoting possible parasite infection. This point has a logical flip side as well: in the context of close interpersonal relationships, the behavioural immune system is likely to be sensitive to superficial cues connoting resistance to parasite infection. A body of research suggests that subjective appraisals of physical attractiveness may serve as such a cue.
We all know that physical attractiveness is a rather important determinant of a person’s sexual allure. That is hardly news. But the story becomes a lot more interesting when we dig deeper 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. strong jaws) that are associated with higher levels of testosterone (Zuk, 2007). We do not have space here to articulate the evolutionary analysis in detail, so we simply emphasise the bottom line – these facial features may also serve as advertisements to potential mates, and what these features advertise is an especially strong immune system.Thus, one reason why subjective appraisals of physical attractiveness are so important in the mating game is that these appraisals are helpful in identifying mates who are likely to be, and to remain, free from parasitic infections. There is some evidence that individuals – especially women – who are 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 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 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.
Good article..........