Oct. 21, 2013 — Bold and
outgoing or shy and retiring -- while many people can shift from one to
the other as circumstances warrant, in general they lean toward one
disposition or the other. And that inclination changes little over the
course of their lives.
Why this is the case and why it matters in a more traditional context
are questions being addressed by anthropologists at UC Santa Barbara.
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."
The Tsimane present a favorable test group because their subsistence
ecology is similar to the way people in developed countries lived for
millennia. "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," said Gurven.
Based on their measurement of different aspects of personality, the
researchers looked at how personality impacted the number of children
men and women 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."
Interestingly, though, Gurven added, the same was not true for women.
"But that wasn't the whole story. 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," he said. Some Tsimane choose to live close to town, near
roads, schools and the various opportunities that accompany the more
urban life, while others live in the remote headwaters, and still others
live in remote forest villages where they're often isolated during much
of the rainy season.
Only among women living in villages near town did personality
associate with higher fertility, Gurven noted. In more remote regions,
the same personality profile had the opposite effect or, in some cases,
no effect on fertility. For men, however, location made no difference.
Wherever they lived, manifesting traits related to extroversion,
openness and industriousness was associated with higher fertility.
So, if higher fertility was the upside of extroversion and other
traits, the researchers wondered what the downside might be. Looking for
potential costs related to these personality traits that associate with
higher fertility, they focused on health and conflicts. Neither, they
discovered, really seemed to be an issue.
"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," Gurven said. "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."
Health was assessed two years after the personality measurements so
there was no possibility that feeling under the weather meant subjects
were more likely to be shy, anxious or dispirited.
Regarding conflicts, the researchers did find that the more
extroverted and open men got in trouble more often. "They did have more
conflicts," Gurven noted. "But most were verbal." And while conflicts
can sometimes escalate into physical confrontation, he added, for the
most part, they don't result in death.
The researchers found no evidence that intermediary levels of
extroversion or other personality traits lead to highest fertility.
Instead, greater levels of these traits associate with higher
reproductive fitness, consistent with the evolutionary model referred to
as directional selection. But personality varied widely between the
sexes -- men scored higher on extroversion, agreableness,
conscientiousness, openness, prosociality and industriousness.
"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."