"The areas were we see the greatest declines in fertility are areas with modern labor markets that have intense competition for jobs and an overwhelming diversity of consumer goods available to signal well-being and social status," says senior author Paul Hooper, an anthropologist at Emory University. "The fact that many countries today have so much social inequality - which makes status competition more intense - may be an important part of the explanation."
The study authors developed a mathematical model showing that their argument is plausible from a biological point of view.
Across the globe, from the United States to the United Kingdom to India, fertility has gone down as inequality and the cost of achieving social status has gone up. "Our model shows that as competition becomes more focused on social climbing, as opposed to just putting food on the table, people invest more in material goods and achieving social status, and that affects how many children they have," Hooper says.
Factors such as lower child mortality rates, more access to birth control and 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."
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Journal Link: Philosophical Transactions of the Royal Society B
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