The Evolution of Human Personality Traits: Outgoing Versus Introverted

Are you bold and outgoing or shy and retiring? People's personalities shift during their lives, but they usually lean one way or the other. Now, scientists are taking a closer look at how these personality traits affect how well we do evolutionarily. The findings could allow researchers to better understand exactly how human personalities managed to develop.

In order to better understand these personalities, the researchers examined over 600 adult members of the Tsimane. This isolated indigenous population is located in central Bolivia. They analyzed personality types and measured fertility and child survivorship. In the end, they found that more open, outgoing and less anxious personalities were associated with having more children--but only among men.

"The idea that we're funneled into a relatively fixed way of interacting with the world is something we take for granted," said Michael Gurven, one of the researchers, in a news release. "Some people are outgoing and open, others are more quiet and introverted. But from an evolutionary standpoint, it doesn't really make sense that our dispositions differ so much, and are not more flexible."

Differences in personalities aren't only seen in humans, either. Scientists have witnessed these types of personality variances in chimps and other species. Yet they've often wondered how dispositional consistency could be favored by selection.

"If personality traits, like extroversion, help you interact easily with bosses, find potential mates and make lots of friends, then why, over time, aren't we extroverted?" asked Gurven. It's a legitimate question, and researchers may just have an answer for it.

It could be that selection pressures vary--whatever is adaptive today might not be so tomorrow, and what is adaptive in one place may not be so in another. Selection pressures can vary between sexes, as well. The most advantageous personality traits for men may not always be so for women. In addition, too much of a good thing is also bad.

"Being more extroverted might also make you more prone to taking unnecessary risks, which can be dangerous," said Gurven.

In fact, the researchers found in this particular study that almost every personality dimension mattered for men. Being more extroverted, open, agreeable and conscientious and less neurotic was associated with having more kids. The same wasn't true for women, though. Only among women living in villages near town did personality associate with higher fertility. In more remote regions, the same personality profile actually had the opposite effect or, in some cases, no effect on fertility.

The findings reveal a little bit more about how personality can affect reproductive outcome. Yet it also shows that the same personality doesn't always work in the same region. Instead, different situations mean than different personalities succeed.

The findings are published in the journal Evolution and Human Behavior.