The Evolution of Personality Differences

Evolutionary psychologist Daniel Nettle wrote an important paper ten years ago, "The Evolution of Personality Variation in Humans and Other Animals" which has not dated. The paper addressed two issues:

• Why is there variation in human personalities?
• What is the evolutionary basis of the 'five factor model' dimensions?

On the first question he has this to say.

"Heritable variation is ubiquitous in natural populations. ... Why does variation persist? The ultimate source of variation is of course mutation. The level of variation found in a population at any point in time reflects the balance between mutation introducing new variants and selection removing them. For a trait influenced by a single gene, selection does not have to be very strong to keep the level of standing variation close to zero, because mutations arise relatively infrequently. However, the rate of appearance of mutations affecting a trait rises directly with the number of genes involved in building it, and selection does not remove them instantaneously. Thus, for a trait affected by many genes, even if selection is strong, there will be a significant amount of standing genetic variation. ...

Varying the level of investment in a quantitative trait is rarely simply advantageous or simply disadvantageous. There are many components to overall fitness, and increasing investment in one trait is usually done at the expense of others. For example, growing large, though possibly beneficial in intraspecific competition, raises metabolic costs and also lengthens the time spent growing and thus delays the onset of reproduction. ...

The extreme case of variable optima is what is known as negative frequency-dependent selection. This describes the situation in which the relative fitness of a trait is high as long as it is rare in the local population but declines as it becomes widespread. It has long been recognized that negative frequency dependence can in theory lead to the maintenance of polymorphism. ...

Variation is a normal and ubiquitous result of the fluctuating nature of selection, coupled with the large numbers of genes that can affect behavior. Frequency-dependent selection, oft discussed as a maintainer of variation, is in fact just a subcase of the more general phenomenon of fluctuating selection. ...

Behavioral alternatives can be considered as trade-offs, with a particular trait producing not unalloyed advantage but a mixture of costs and benefits such that the optimal value for fitness may depend on very specific local circumstances. With these generalizations in mind, I now turn to the consideration of personality variation in humans."

We may observe that every form of human society from hunter-gatherer through pastoral, agricultural and mercantile-capitalist has required a division of labour where different personality types are optimal. Not everyone can be the big chief or be a compliant follower, not everyone can focus on competence in tool-making, not everyone can be a specialist in interpersonal diplomacy.

I summarise Professor Nettle's views on the evolutionary validity of the five factors below (his second question). The Myers-Briggs Type Theory dimensions are shown in brackets. I have edited-out the original inline references for readability - refer to the original paper (PDF) for the full text.

1. Extraversion-Introversion (E-I) 

"Extraversion is strongly and positively related to number of sexual partners which, for men in particular, can increase fitness. High scorers are also more likely to engage in extra-pair copulations or to terminate a relationship for another. This may lead to their securing mates of higher quality than those secured by individuals who are more constant in their choice of partners. The benefits of extraversion are not limited to mating, as extraverts,or those high on the closely correlated trait of sensation seeking, initiate more social behavior and have more social support than others. Moreover, they are more physically active and undertake more exploration of their environment.

However, in pursuing high sexual diversity, and high levels of exploration and activity in general, extraverts also expose themselves to risk. Those who are hospitalized due to accident or illness are higher in extraversion than those who are not, and those who suffer traumatic injury have been found to be high in sensation seeking. High extraversion or sensation seeking scorers also have elevated probabilities of migrating, becoming involved in criminal or antisocial behavior, and being arrested. All of these are sources of risk, risk that in the ancestral environment might have meant social ostracism or death.Moreover, because of their turnover of relationships, extraverts have an elevated probability of exposing their offspring to stepparenting, which is a known risk factor for child well-being."

2. Neuroticism-Emotional Stability (Turbulent-Assertive as defined here)

"The neuroticism personality axis is associated with variation in the activity levels of negative emotion systems such as fear, sadness, anxiety, and guilt. The negative effects of neuroticism are well-known in the psychological literature. High neuroticism is a strong predictor of psychiatric disorder in general, particularly depression and anxiety. Neuroticism is also associated with impaired physical health, presumably through chronic activation of stress mechanisms. Neuroticism is a predictor of relationship failure and social isolation. A much more challenging issue, then, is finding any compensatory benefit to neuroticism.

However, given the normal distribution observed in the human population, and the persistence of lineages demonstrably high in the trait, such a benefit seems likely. Studies in nonhuman animals, such as guppies, suggest that vigilance and wariness are both highly beneficial in avoiding predation and highly costly because they are quickly lost when predation pressure is absent. In ancestral environments, a level of neuroticism may have been necessary for avoidance of acute dangers. Anxiety, of which neuroticism can be considered a trait measure, enhances detection of threatening stimuli by speeding up the reaction to them, interpreting ambiguous stimuli as negative, and locking attention onto them.

Because actual physical threats are generally attenuated in contemporary situations, the safety benefits of neuroticism may be hard to detect empirically. However, certain groups who take extreme risks, such as alpinists and Mount Everest climbers, have been found to be unusually low in neuroticism. Given the high mortality involved in such endeavors (around 300 people have died in attempting Everest), this finding suggests that neuroticism can be protective."

3. Openness-Concreteness (N-S)

"The trait of openness to experience again seems, at first blush, to be an unalloyed good. Openness is positively related to artistic creativity. According to Miller’s cultural courtship model, creative production in artistic domains serves to attract mates, and there is evidence that women find creativity attractive, especially during the most fertile phase of the menstrual cycle, and that poets and visual artists have higher numbers of sexual partners than controls. The core of openness seems to be a divergent cognitive style that seeks novelty and complexity and makes associations or mappings between apparently disparate domains. Though such a cognitive style might appear purely beneficial, it is conceptually very similar to components of schizotypy, or proneness to psychosis. ...

Thus, openness and its covariates are associated with damaging psychotic and delusional phenomena as well as high function. Openness itself has been found to be associated with depression, as has a high score on the Unusual Experiences scale. Thus, the unusual thinking style characteristic of openness can lead to nonveridical ideas about the world, from supernatural or paranormal belief systems to the frank break with reality that is psychosis."

4. Conscientiousness-Spontaneity (J-P)

"The remaining two personality domains, conscientiousness and agreeableness, are often thought of as being unalloyed in their benefits, because they are generally negatively related to measures of delinquency and antisocial behavior. However, it is important not to conflate social desirability with positive effects on fitness. Natural selection favors traits that increase reproductive success, including many cases in which this success comes at the expense of other individuals. It is likely that fitness can be enhanced by a capacity to demand a free ride, break rules, and cheat on others under certain circumstances.

Conscientiousness involves orderliness and self-control in the pursuit of goals. A by-product of conscientiousness is that immediate gratification is often delayed in favor of a longer term plan. This leads, for example, to a positive association of conscientiousness with life expectancy, which works through adoption of healthy behaviors and avoidance of unhygienic risks. Very high levels of traits related to conscientiousness - moral principle, perfectionism, and self-control - are found in patients with eating disorders and with obsessive-compulsive personality disorder.

Though some obsessional individuals can be very high achievers in the modern context, it is not evident that their fitness would always have been maximal in a variable and unpredictable ancestral environment. Their extreme self control not only may be damaging, as their routines become pathological, but may lead to the missing of spontaneous opportunities to enhance reproductive success. Highly conscientious individuals have fewer short-term mating episodes and will forgo opportunities to take an immediate return that may be to their advantage. Adaptations that orient the organism toward working for long-term payoffs will tend to have the effect of reducing the opportunistic taking of immediate ones."

5. Agreeableness-Tough-Mindedness (F-T)

"Agreeableness, with its correlates of empathy and trust, is also generally seen as beneficial by personality psychologists, and its absence is associated with antisocial personality disorder. Agreeableness is strongly correlated with Baron-Cohen’s empathizing scale, which is in turn argued to measure theory of mind abilities and the awareness of others’ mental states.

Several evolutionary psychologists have argued plausibly that as a highly social species, humans have been under strong selection to attend to and track the mental states of others. Others have noted that we seem to be unique among mammals in the extent of our cooperation with unrelated conspecifics. Inasmuch as agreeableness facilitates these interactions, it would be highly advantageous. Agreeable individuals have harmonious interpersonal interactions and avoid violence and interpersonal hostility. They are much valued as friends and coalition partners. ...

Although this may be true, a vast literature in theoretical biology has been devoted to demonstrating that unconditional trust of others is almost never an adaptive strategy. Across a wide variety of conditions, unconditional trusters are invariably outcompeted by defectors or by those whose trust is conditional or selective. ...

Though it is an uncomfortable truth to recognize, it is unlikely that fitness is unconditionally maximized by investing energy in positive attention to others. Instead, though an empathic cognitive style may be useful in the whirl of social life, it may have costs in terms of exploitation or inattention to personal fitness gains.

Moreover, sociopaths, who are low in agreeableness, may at least sometimes do very well in terms of fitness, especially when they are rare in a population. The balance of advantages between being agreeable and looking after personal interests will obviously vary enormously according to context. For example, in a small isolated group with a limited number of people to interact with and a need for common actions, high agreeableness may be selected for. Larger, looser social formations, or situations in which the environment allows solitary foraging, may select agreeableness downward."

Professor Nettle leaves us with this useful summary picture.

Nettle agrees that his ideas are speculative and are intended to propose hypotheses for future research. In fact it has proved hard to directly identify genotype correlates to psychological traits. Genes code for brain structure and process-regulation in a highly-indirect way. The missing level of analysis is in the variant brain architectures and operating modes (such as neurotransmitter levels) which underlie individual personality traits and differences - these seem to be largely under genetic control.

Thankfully, there's a lot of research into these areas at the moment.