Using fertility and child survivorship as their main measures of reproductive fitness, the researchers studied over 600 adult members of the Tsimane, an isolated indigenous population in central Bolivia, and discovered that more open, outgoing -- and less anxious -- personalities were associated with having more children -- but only among men.
Their findings appear online in the journal Evolution and Human Behavior
"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, UCSB 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?"
Given the variability in personality, a question then is how that variability is maintained 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. Successful behavioral strategies with genetic underpinnings -- and behavioral genetics has demonstrated relatively high heritability for personality variation -- often increase in frequency over time, and therefore reduce variation over many generations.
One reason might be that selection pressures vary -- whatever is adaptive today might not be so tomorrow, and what is adaptive in one place might 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. A second reason could be the idea that too much of a good thing is bad. "Being more extroverted might also make you more prone to taking unnecessary risks, which can be dangerous," Gurven said.
Gurven and his team wanted to examine the personality measures they had on the Tsimane adults and determine 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," 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."