Why Neurotics Haven't Died Out

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How many children people have can be predicted by their personality, at least in certain populations with high birth rates, a new study finds.

For women, high levels of neuroticism, a personality trait meaning a tendency to feel anxious and moody, was associated with having more children. Not surprisingly, extroversion was linked to more offspring for men.

The study, conducted in Senegal, is one of only a handful to address the questions: Why do human personalities differ to begin with, and how has evolution shaped these traits? The Senegalese population studied is thought to more closely resemble the environment in which humans evolved.

It suggests why some seemingly disadvantageous personality traits, such as neuroticism, may have stuck around over the course of evolution. And it indicates that there is more to personality than just the ability to make a good first impression.

"You mustn’t think of personality traits only in terms of how they affect your happiness, or how other people might perceive you," said study researcher Markus Jokela, of the University of Helsinki in Finland. "When you're considering the evolutionary origins of personality, then obviously reproductive success and behaviors related to reproductive success are very very important."

Personality and fertility

Most past studies focused on the evolution of human personality have been carried out in Westernized countries — populations in which access to birth control has allowed couples to have fewer children; and quality hospital care means fewer die young, both factors that are not characteristic of pre-industrial societies.

Jokela and his colleagues studied four villages in rural Senegal that practice polygamy and have higher birth rates and mortality rates than those of more developed countries.

They used questionnaires to assess the personality traits of parents in 65 families. They tested for the so-called "Big Five" personality dimensions — extroversion, neuroticism, agreeableness, openness and conscientiousness — traits considered the basic components to anyone's personality.

Women with above average levels of neuroticism had 12 percent more children than those with below-average levels.

Men with above average extroversion (sociable and outgoing) had a 31-percent higher chance of attaining a high social status than those with below average extroversion. The extroverts were also 40 percent more likely to have more than one wife, and had 14 percent more children than those with below-average levels.

Neuroticism had a drawback when it came to the physical fitness of the children. Young children (ages 0 to 5) of mothers with high levels of neuroticism had a BMI that was 18 percent lower than children of mothers with low neuroticism levels. Low BMI indicates the children could be malnourished. However, this result was only true of families with low social rank.

"You would expect that fewer of those children, offspring of highly neurotic women, would survive in the long run," Jokela said.

Indeed, neuroticism appeared to be a double-edged sword in terms of reproduction, with the optimal amount being somewhere in the middle. For women, an intermediate level of neuroticism was associated with the maximum reproductive success, meaning having the most children that you would expect to live to adulthood.

Many have suspected that personality provides some sort of trade-off between the number of offspring and their health, but the current study is one of the first to provide experimental evidence of such a trade-off, Jokela said.

Why does neuroticism mean more kids?

The scientists aren't sure why neuroticism leads to more children in this society, but they have some speculations. Neuroticism has been linked to competitiveness, and it's possible that, in a society where having offspring is highly valued, the neurotic women have more babies to out-compete other wives.

Also, neurotic women might be more sexually active than others, which goes along with previous studies that have linked the personality trait with attachment anxiety (think clingy partner) and high sexual motivation in Westernized countries, the researchers say. In populations with less access to birth control, a high sex drive could translate into more children.

Interestingly, studies in developed countries have found neuroticism actually decreases the probability of having large families, or has no effect at all. This suggests the impact of a personality trait on fertility can depend on the cultural context.

The researchers also don't know why children of neurotic women were in poorer physical shape. But it could be that women with more neuroticism have difficulties managing larger families, Jokela said.

The fact that the men's personality didn't impact the children's physical shape could reflect the different roles men and women have in child rearing in this society, Jokela said, with women having a larger role.

The results were published June 7 in the journal Proceedings of the National Academy of Sciences.

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