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Pregnancy Timeline by SemestersFetal liver is producing blood cellsHead may position into pelvisBrain convolutions beginFull TermWhite fat begins to be madeWhite fat begins to be madeHead may position into pelvisImmune system beginningImmune system beginningPeriod of rapid brain growthBrain convolutions beginLungs begin to produce surfactantSensory brain waves begin to activateSensory brain waves begin to activateInner Ear Bones HardenBone marrow starts making blood cellsBone marrow starts making blood cellsBrown fat surrounds lymphatic systemFetal sexual organs visibleFinger and toe prints appearFinger and toe prints appearHeartbeat can be detectedHeartbeat can be detectedBasic Brain Structure in PlaceThe Appearance of SomitesFirst Detectable Brain WavesA Four Chambered HeartBeginning Cerebral HemispheresFemale Reproductive SystemEnd of Embryonic PeriodEnd of Embryonic PeriodFirst Thin Layer of Skin AppearsThird TrimesterSecond TrimesterFirst TrimesterFertilizationDevelopmental Timeline
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Home | Pregnancy Timeline | News Alerts |News Archive Oct 28, 2013


Tsimane mother and her children.

Image Credit: Emily Miner

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Anthropologist explain evolutionary benefit of personality traits

Bold and outgoing or shy and retiring, while many people shift from one to the other as circumstances warrant, in general they lean toward one disposition more that the other. 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, researchers studied over 600 adult members of the Tsimane, an isolated indigenous population in central Bolivia.

They discovered 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.

Gurven continued: "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.

"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 help you interact easily with bosses, find potential mates and make lots of friends, then why, over time, aren't we all 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.

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.

Gurven explained: "Considering the evolutionary adaptiveness of a trait like personality can be problematic in modern developed societies because of the widespread use of contraception. 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. "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.

"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."

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. 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.

If higher fertility is the upside of extroversion and other traits, the researchers wondered what the downside might be. Looking for potential costs related to personality traits associated with higher fertility, researchers focused on health and conflicts. Neither 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 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. And while conflicts can sometimes escalate into physical confrontation, Gurven added, for the most part, they don't result in death.

Researchers found no evidence that intermediate levels of extroversion or other personality traits lead to the highest fertility. Although 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.

The maintenance of personality variation remains an unexplained puzzle in evolutionary biology. Despite evidence among non-humans that personality variation affects fitness, few data exist to assess the personality–fitness relationship in humans. Among Tsimane forager–horticulturalists (n=632), we test whether personality traits (assessed using a 43-item Big Five Inventory administered orally in native language) predict fertility, offspring survivorship, age of first reproduction, and other fitness correlates (extramarital affairs, conflicts, social visitation, food production, and several health measures). Among men, several personality factors associate with higher fertility, more time spent producing food and social visitation. Among women, the relationship between personality and fitness varies across regions of Tsimane territory. The only case of an intermediate personality level associated with highest fitness was found for Industriousness in men. We find that personality factors positively associated with fitness do not associate with greater health costs, although greater Extraversion and Openness may lead to more conflicts among men. Factor heritability ranges from 60% for Prosociality and Extraversion to 8% for Neuroticism. We interpret our results in light of evolutionary models that explain maintenance of personality variation, including incomplete directional selection, mutation–selection balance, condition-dependent reaction norms and fluctuating selection based on sex or spatial variability in selection pressures.

The paper's other co-authors include Christopher von Rueden of the Univeristy of Richmond; Hillard Kaplan of the University of New Mexico, Albuquerque and co-director of the Tsimane Health and Life History Project; Jonathan Stieglitz of the University of New Mexico, Albuquerque; and Daniel Eid Rodriguez of the Universidad Mayor de San Simón, Cochabamba.

Original press release: http://www.ia.ucsb.edu/pa/display.aspx?pkey=3143