In a recent post, I argued that extraverts are better at managing stress in modern environments although vulnerable to boredom. In the evolutionary past, they likely relieved the boredom by having affairs. Did this improve their reproductive success?

Introversion-Extraversion as a Balanced Polymorphism

Some people are extreme introverts. Others are extreme extraverts. Most of us are somewhere in between. Extraversion is highly heritable, implying that it has a genetic basis.

The fact that people still vary on the extraversion continuum implies that natural selection could not deliver a clear verdict for extraversion over introversion.

The persistence of both introversion and extraversion in modern populations means that it is a balanced polymorphism. So the average benefits of being an introvert must be roughly equivalent to the average benefits of being an extravert.

The problem about this thesis is that the benefits of extraversion seem stronger. In particular, there is evidence that extroverts are better at handling stress, thereby conferring a probable advantage for psychological, and bodily health.

Extraversion as a Sexual Phenotype

Extroverts are more sociable and outgoing so that they are in a better position to meet members of the opposite sex. As one might expect they have larger social networks than introverts who characteristically spend more time engaged in solitary pursuits.

Evidence from both indigenous populations, such as the Tsimane of Bolivia, and modern societies shows that extroverts have more children than average (1,2). This is a substantial advantage for men, and a modest advantage for women.

If extraversion provides a clear reproductive fitness advantage, then one might expect that introversion would be strongly selected against, and therefore extremely rare. Yet, that is not the case.

Why Introversion Persists: Risk Aversion and Nurture?

The simplest explanation for the persistence of introversion is that extraversion has a down side. Extraverts may be more effective at enhancing their own social status by virtue of their extensive social networks. In the process, though, they are more likely to get into arguments and have a higher risk of violent injuries (2). Extraverts are also at greater risk of contracting infectious diseases. Moreover, their social drive might predispose them to neglecting their families so that their marriages crumble and their children are at greater risk of harm.

These factors seem to provide a real advantage for introverts, even if they are counterbalanced by the greater capacity of extraverts to handle psychological stress with associated health advantages.

Despite these qualifications, the evidence is clear enough. Extraverts have a substantial reproductive advantage. By that logic, genes predisposing people to extroversion should long ago have beaten out those that predispose to introversion. There should be almost no introverts today.

How can one reconcile the reproductive advantage of extraverts with this lack of evidence for natural selection acting to reduce introversion? One possibility is that there are no “extraversion genes” on which natural selection could act.

The Search for Extraversion Genes

Yes, it is true that extraversion is strongly heritable (at around 60 %) just as some other personality traits are. (2). Oddly, this does not mean that there are genetic variants that predispose either to extraversion or introversion.

Thanks to the success of the Human Genome Project, it is now possible to study genotypes in detail and analyze them for correlations with personality traits using fancy statistical procedures. When this was done, researchers drew a complete blank (2). There is no evidence of particular gene variants underlying extraversion which would account for the persistence of introversion in the world despite extraverts having more children. In other words, natural selection has nothing upon which to act.

So how could extraversion be so substantially heritable?

Researchers have come up with an explanation that interprets extraversion as more of a side product of being strong and healthy than some trait that may be inherited like eye color.

Extraversion as a Response to Health and Strength

How can you have high heritability for extraversion without genetic alleles that distinguish introverts from extraverts? One hypothesis is that social assertiveness is dependent on being in good health. It is a facultative adaptation that gets expressed only if a person is in good condition, meaning that they are strong, well-nourished, and physically attractive.

This principle likely holds for all social animals: those that are in poor health avoid social interactions. Farmers know that a sheep who is separated from the flock is likely to be sick. By the same reasoning, individuals who are in better health are likely to be more actively engaged in social interactions and more willing to assert their dominance in interactions with others.

Similarly, people who are strong, healthy, and physically attractive, are more likely to enter the rough and tumble of social interactions with relative strangers. Researchers find that extraverts are, more physically imposing, and more physically attractive, than introverts both in modern societies and amongst indigenous residents of the Amazon region (2).

So the logic is that they are better equipped to assert themselves in relationships with others, even if this entails some risks of conflict and aggression.

If these ideas are correct, then extraversion itself is not associated with genes that are subject to natural selection. Instead,what is inherited are the thousands of genes affecting disease resistance, growth, physical attractiveness and strength. These traits contribute to being in good physical condition. People in good physical condition behave in more extroverted ways.

If this condition-dependent hypothesis turns out to be correct, it will force us to think of personality as a lot more fluid than was true before. One implication is that a person who overcomes serious illness is liable to behave in more extroverted ways. Likewise, the strongest child in a group of relative weaklings may be more socially assertive than if they were in the company of athletes.

References

1: Gurven, M., et al. (2014). The evolutionary fitness of personality traits in a small-scale subsistence society. Evolution and Human Behavior, 35, 17-25.

2: Lukaszewski, A. W., and von Rueden, C. (2015). The extroversion continuum in evolutionary perspective. Personality and Individual Differences, 77, 186-192.