Open, outgoing men from an isolated indigenous population in central Bolivia were more likely to have more children than their shy counterparts, a new study found.
The study, published in the journal Evolution and Human Behavior, examines the evolutionary purposes and effects behind different personality traits.
"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, a University of California, Santa Barbara professor of anthropology and the paper's lead author. "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."
One question the researcher had in carrying out the study is why variability has persisted over time.
"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?" Gurven asked.
One possibility, he hypothesizes, is that too much of a good thing can be bad.
"Being more extroverted might also make you more prone to taking unnecessary risks, which can be dangerous," Gurven posited.
To find out what consequences may unfold from one personality versus another, the researchers turned to the Tsimane, a group of people that in many ways continues to live much like their ancestors before them.
"Considering the evolutionary adaptiveness of a trait like personality can be problematic in modern developed societies because of the widespread use of contraception," Gurven explained. "In all animals -- including humans -- the better condition you're in, the more kids you have. 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."
A Tsimane woman gives birth an average of nine times during her lifetime, offering "a ripe kind of population for trying to look at personality," Gurven said.
The researchers first measured different aspects of personality, and then examined how those differences influenced the number of children men had.
"And what we found was that almost every personality dimension mattered for men, and it mattered a lot," Gurven said. "Being more extroverted, open, agreeable and conscientious -- and less neurotic -- was associated with having more kids."