Why Are There Introverts and Neurotics?

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When it comes to human personality, there are some standard pickles. For instance, when people learn about the broad and basic trait dimension of introversion/extraversion, extraversion seems to get all the glory. Extraverts come across as fun and sociable. They have relatively large social networks. And they have more sexual partners, on average, than do introverts (Nettle & Clegg, 2008).

The dimension of neuroticism/emotional-stability is similar. Those who are emotionally stable get all the glory. They report having relatively little in the way of stress or depression. They make for better romantic partners. And their social relationships tend to be pretty stable. Who would want to be neurotic?

Basic Personality Traits As Normally Distributed

The plot thickens.

Basic personality traits, such as extraversion and emotional stability, tend to emerge as normally distributed across various human populations. In short, most people tend to score as close to average on these dimensions, while a small subset of any population will score as extremely high (e.g., very extraverted) or as extremely low (e.g., very introverted).

From an evolutionary perspective, this fact gives us reason for pause. If extraversion is so much more beneficial (at least superficially) than is introversion, then why isn't everyone an extravert? If being relatively extraverted is more evolutionarily adaptive than being introverted, then wouldn't natural selection weed out the introverts?

Same goes for the dimension of emotional stability, right? If emotional stability is so great compared to neuroticism, you'd think that natural selection would ultimate lead to populations comprised exclusively of emotionally stable individuals.

But, in fact, that's not what we see. With each of the Big Five personality trait dimensions (see Costa & McCrae, 1985), which seem to be foundational in defining human personality, distributions of scores across human populations tend to map onto normal distributions.

What's going on?!

Balancing Selection and the Basic Personality Traits

When it comes to natural selection, it is not always the case that some trait has only a single variant that is adaptive. In fact, it's often the case that multiple variants of a trait can be adaptive, largely based on the relative prevalence of different variants that are out there in the population already. If the population is filled with extraverts, then there may be fitness-related advantages to being an introvert. In such a scenario, the relative prevalence of the different phenotypes, or trait variants, would ultimately balance out mathematically. Generally, this means that the costs and benefits of one variant (from an evolutionary perspective) would roughly equal the costs and benefits of an alternative variant. This is what we mean by the term balancing selection.

In 2008, Dan Nettle and Helen Clegg published a great paper on the Big Five personality traits conceptualized through the lens of balancing selection. They basically argue that each of the Big Five foundational traits (including extraversion/introversion, emotional-stability/neuroticism, conscientious/disorganized, agreeableness/disagreeableness, and open-minded/closed-minded) has a distinct set of evolutionarily relevant costs and benefits. And for this reason, we tend to see a healthy balance of these different personality trait variants across a broad array of human populations.

Here is brief summary of the evolutionary costs and benefits that Nettle and Clegg (2008) present for each of the Big Five traits.

Extraversion/Introversion: Extraverts enjoy such benefits as increased social status, increased social network size, increased number of sexual partners, and being rated as relatively attractive as mates. On the downside, they are more likely to die young, they have increased risk of accidents, and they are more likely to experience harsh relationship dissolution related to such things as infidelity.

Emotional Stability/Neuroticism: Those who are emotionally stable tend to be relatively attractive as romantic partners, and their romantic relationships tend to be relatively satisfying. They also report relatively little stress or anxiety. On the other hand, they are less vigilant than are their neurotic counterparts, making them less sensitive to cues regarding things like infidelity and relationship desertion.

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Conscientious/Disorganized: We tend to report preferring relatively conscientious mates. And those who score high on measures of conscientiousness tend to be rated as relatively trustworthy and faithful. On the downside, conscientiousness seems to go hand-in-hand with reduced mating opportunities, which is a clear evolutionary cost.

Agreeableness/Disagreeableness: We tend to prefer agreeableness in all kinds of social partners. And agreeable people enjoy such benefits as having others cooperate with them and smooth interactions in various kinds of relationships. However, agreeableness can come with certain social costs. For instance, agreeableness is sometimes associated with low social status.

Open-mindedness/Closed-mindedness: Open-mindedness tends to be relatively attractive compared to closed-mindedness. And the open-minded among us tend to score relatively high on markers of creativity. This said, there seem to be costs associated with being too open-minded. Open-mindedness seems to correspond to a vulnerability for various forms of mental illness.

Bottom Line

What is the "best personality" to have? The answer is this: It depends! Across human populations around the world, basic personality traits generally show normal distributions—most scores are near the mean. Few scores are extremely high or low.

Why does such extreme variability in personality exist? From an evolutionary perspective, one way to answer this question is found in the concept of balancing selection. Different trait variants have their own evolutionary costs and benefits. An extremely extraverted individual might be more likely to secure a viable mate, which is a clear evolutionary benefit, but that same extravert may also be more likely to fall off a cliff. And that is what we call a clear evolutionary cost.

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