Personality And Evolution - The Shy Guy Doesn't Get The Girl
October 23, 2013

UCSB Anthropologist: Extroverted Men Make More Babies

Alan McStravick for - Your Universe Online

If you’ve ever learned how we all possess different personality types, then chances are you are also familiar with the Meyers-Briggs Type Indicator (MBTI) personality assessment. The MBTI aims to help individuals understand their specific personality type so they can employ what they learn to help them make the best of their careers, relationships and daily lives in general. But what if your personality type was directly responsible for the evolutionary success of both you and your future progeny?

A new study has taken the idea of personality type and applied it to fertility and child survival as a way to measure reproductive fitness. In order to carry this study out, the team studied more than 600 adult members of the Tsimane. The Tsimane are an indigenous people living in central Bolivia. What the team was able to determine is that being more open and outgoing was directly associated with having more children. Most interesting, however, was this finding really only applied to 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,  University of California - Santa Barbara professor of anthropology and the paper's lead author. Gurven is also co-director of the University of New Mexico-based Tsimane Health and Life History Project. "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.

"Wouldn't it be great to be more extroverted at an important party, more conscientious when you're on the clock at work, less anxious when talking to a potential date?" Gurven continued. "Differences in personality and their relative stability are not unique to humans, and have now been studied in many species, from ants to primates. How could dispositional consistency be favored by selection?"

What Gurven is alluding to is the fact that if one personality type seems to produce better evolutionary results, then how do the less beneficial types maintain their genetic stronghold within a species? “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?” he asked. One would expect, given the high heritability for personality variation, the more beneficial trait would, through an increase in its frequency, genetically decimate the less beneficial trait over several generations.

Of course, looking at it from that single perspective is very elementary. It doesn’t take into account the variability of certain selection pressures. It is these selection pressures, which can vary from generation to generation and even between the sexes themselves, that could be responsible for the less beneficial introverted personality pursisting in a population. Gurven also suggests a second possibility for the perpetuation of less beneficial personality types. “Being more extroverted might also make you more prone to taking unnecessary risks, which can be dangerous.”

With those two ideas in mind, Gurven and his team set out to examine the personality measures they had on the Tsimane adults. Ultimately, they wanted to learn what consequences might result from one personality over another. “Considering the evolutionary adaptiveness of a trait like personality can be problematic in modern developed societies because of the widespread use of contraception. In all animals – including humans – the better condition you’re in, the more kids you have,” Gurven stated. “And for humans in more traditional environments, like the Tsimane, the higher your status, the better physical condition you’re in, the earlier you might marry, and the higher reproductive success you’re likely to have.”

The reason the team felt the Tsimane were the best candidates to study resides in the fact these indigenous people maintain a subsistence ecology like other populations in developed countries have done for millennia. According to Gurven, “It’s a high fertility population – the average woman has nine births over her lifetime – and a ripe kind of population for trying to look at personality.”

And what the team found was both interesting and important. “What we found was that almost every personality dimension mattered for men, and it mattered a lot. Being more extroverted, open, agreeable and conscientious – and less neurotic – was associated with having more kids.”

However, this fact didn’t necessarily hold true for the women of the Tsimane. “Because we had a large number of test subjects, we could look at whether the relationship between personality and reproduction varied across different regions of the Tsimane territory,” Gurven noted. Many sub-groups of the Tsimane prefer a more rural existence while others prefer the urban lifestyle, living closer to town, with its roads, schools and other opportunities.

For women, the team discovered personality was only a factor in a higher fertility rate for those living in the villages near town. The further away from town they traveled, the more it was noted that the personality profile actually exhibited an opposite effect or, in some cases, no effect on fertility. Extroverted men, on the other hand, enjoyed higher reproductive success regardless of where they lived.

The next natural step for the research team was, after determining the benefit to extroversion, to try to understand what the detriment of an outgoing personality might be. To achieve this goal, the team focused on health and conflict. Surprisingly, neither seemed to factor into lower fertility or offspring survivability.

Gurven explained, "You might think that folks putting themselves out there all the time would be getting sick more often because of greater pathogen exposure or from taking risks, but we didn't find much evidence that they were sicker. If anything, they were consistently healthier. Which actually makes sense when you consider that of people who are in good condition in general are both healthier and more likely to be outgoing."

With regard to the conflict factor, the team noted extroverted men were more likely to get themselves into a bit of trouble than those regarded as introverted. “They did have more conflicts, but most were verbal.” Even if a conflict ultimately escalated into a physical confrontation, it rarely resulted in death, thereby playing no significant role in a diminished overall fertility.

"That the relationship between personality and fitness varies by sex and geographical region supports the view that fluctuating selection pressures may help maintain variation in personality," said Gurven. "Selection pressures may vary over time as well. Indeed, the environment Tsimane face today may be somewhat novel. The annual growth rate of the Tsimane population over the last several decades is almost four percent –– meaning the population doubles every 17 years –– which suggests pioneer-like conditions. Greater market access, schooling and other opportunities are producing further changes in Tsimane society."

Gurven and his team have managed to put scientific data behind a truth understood by many. Those among us who are more boisterous, outgoing and affable enjoy more successful relationships, especially as they pertain to propagating a lineage. Simply stated: the shy guy doesn’t get the girl.

Their findings appear online in the journal Evolution and Human Behavior.

Image 2 (below): A Tsimane mother and her children. Credit: Emily Miner