Fertility

Birth control: the choice to delay childbirth to get a higher education are also associated with declining fertility. "While these factors are very important they are insufficient to explain the drops in family sizes that we are seeing," Hooper says. In addition to Hooper, the study authors include anthropologists Mary Shenk, from the University of Missouri, and Hillard Kaplan, from the University of New Mexico. They are pioneers in an emerging field of "computational anthropology," which blends methods from biology, economics, computer science and physics to answer fundamental questions about human behavior.

The study is featured in a special issue of the Philosophical Transactions of the Royal Society B, devoted to how evolutionary approaches can help solve the puzzle of why human fertility varies substantially.

Hooper first became intrigued by variability in human fertility while researching the Tsimane indigenous people of Bolivian Amazonia. The Tsimane (pronounced chee-mahn-AY in Spanish) are foragers and horticulturalists who live in small, isolated communities along the Maniqui River in the Amazonian rainforest.

"In a hunter-gatherer society, parents have a limited number of things available to invest in: Food, clothing and shelter," Hooper says. "The average Tsimane family has nine children and they can provide these basic needs for all of them."

Hooper noticed a pattern, however, when Tsimane families leave the rainforest and move closer to Spanish-speaking towns where they come into contact with market economies and industrialized goods. "When they start getting earnings for the first time, they spend money on things you wouldn't really expect, like an expensive wristwatch or a nylon backpack for a child attending school, instead of sending them with a traditional woven bag," Hooper says. "I got the impression that these things were largely symbolic of their social status and competence."

The Tsimane family size also tends to drop when they move closer to town: From eight or nine children in remote villages, to five or six in villages near town, to three to four in the town itself, he adds.

Hooper hypothesizes that a similar pattern plays out as societies develop from mainly agrarian to more urban and affluent. "In my grandparents day, it took a lot less investment to be respectable," he says. "It was important to have a set of good clothes for church on Sunday but you could let the kids run around barefoot for the rest of the week."

Today, however, keeping up with the Jones has become much more complicated - and expensive.

"The human species is highly social and, as a result, we appear to have an ingrained desire for social standing," Hooper says. "The problem is that our brains evolved in a radically different environment from that of the modern world. Evolution didn't necessarily train us very well for the almost infinite size of our communities, the anonymity of many of our interactions and the vast numbers of goods that we can use to signal our status. Our evolved psychology may be misfiring and causing us to overinvest in social standing."