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.

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.