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For many European explorers, the New World was a blank slate upon which they could write anew, if only it weren’t for the millions of people who already lived there. In 1633, the French missionary Paul Le Jeune wrote from northeastern Canada to his Jesuit order about the great difficulties he had in converting the indigenous Montagnais people to Christianity. “The inconstancy of marriages and the facility with which they divorce each other, are a great obstacle to the Faith of Jesus Christ,” he complained. However, what was even more alarming to Le Jeune’s Christian sensibilities was the tendency of married women and men to take lovers, many of whom would openly raise together the children from these affairs. In one telling exchange with the village shaman, Le Jeune condemned such “savage” and “licentious” behavior:
I told him that it was not honorable for a woman to love any one else except her husband; and that, this evil being among them, he himself was not sure that his son, who was there present, was his son. He replied, “Thou hast no sense. You French people love only your own children; but we all love all the children of our tribe.” I began to laugh, seeing that he philosophized in horse and mule fashion.
The anthropological literature has a rich tradition of privileged white men expressing shock and indignation over the sexual behavior of other cultures. However, even from the field’s inception, it was well understood that Western-style monogamy was anything but the norm. The American ethnographer Lewis Henry Morgan, for example, wrote in his 1877 book Ancient Society that a flexible marriage system was common for “primitive” societies and was one that “recognized promiscuity within defined limits.” Morgan’s work was so highly influential at the time that Darwin was forced to admit in The Descent of Man, “It seems certain that the habit of marriage has been gradually developed, and that almost promiscuous intercourse was once extremely common throughout the world.”
Despite this early acknowledgement that human societies had a range of approaches to sexual fidelity, few researchers chose to pursue the question from a woman’s perspective. As a result, as late as 1982, Donald Symons, an anthropologist and early founder of evolutionary psychology, could write that there was “dubious evidence that this [assertive sexual female] nature exists and no evidence that women anywhere normally tie up multiple male parental investments.”
The village networks in the Omuhonga basin of northwestern Namibia would prove such ideas about female agency wrong. It was here, surrounded by giant acacia trees, that anthropologist Brooke Scelza interviewed married women among the Himba, seminomadic pastoral people who live almost exclusively on livestock. These Himba women, their skin and elaborate braids beautifully decorated in red pigment made from crushed ochre and animal fat, would be entered into arranged marriages at a young age. However, as Scelza discovered, while their husbands traveled long distances managing the herds, female adultery was commonplace back home. Out of 110 women interviewed, fully one-third said that they sought out extramarital affairs that resulted in the birth of at least one child. Because there is no social stigma attached to these liaisons in Himba society, both women and men discuss them openly. (Divorce can likewise be initiated by either party.) As a result, according to Scelza’s analysis published in the journal Biology Letters in 2011, “women who had at least one extra-pair birth have significantly higher reproductive success than women with none.”
Of course, this was certainly not the first time that extra-pair paternity had been connected to female reproductive success. Previous studies have reported evidence of female infidelity in small-scale societies such as the !Kung of South Africa, the Ekiti of Nigeria, the Vanatinai of New Guinea, the Tiwi of Northern Australia, the Tsimane of Bolivia, and the Yanomami of Brazil. In addition, 53 societies can be classified as having systems of “informal polyandry” in which women have simultaneous sexual relationships with more than one man. In many South American societies, such as the Ache, Bari, Canela, Mundurucu, and Mehinaku, it is believed that it takes the semen of several men to produce a baby. In two of these “partible paternity” societies, the Ache and Bari, children with more than one father were found to have lower mortality and improved nutrition due to a greater level of provisioning. When anthropologists Kim Hill and A. Magdalena Hurtado asked 321 Ache about their kinship information, the total included 632 fathers, or an average of two “fathers” each. This is perhaps not so different from the common situation of American children who receive support from both their biological father and current stepfather. As long as the biological father contributes support, such children might well gain by having two fathers.
While a great diversity of sexual norms exist around the world, ranging from strictly enforced monogamy to polyamory, according to Scelza’s new study there are two environmental contexts where women commonly choose multiple partners. The first is where women have more material support from their kin or economic independence from men more generally. This may explain why multiple mating is most common among small-scale matrilocal societies (in which women remain in their home village after marriage), such as the partible paternity societies of South America or the Mosuo of China. It may also explain why female infidelity has increased in Western societies as women have gained greater political and economic independence. (For example, Iceland was ranked first in gender equality by the World Economic Forum in 2013 at the same time that 67 percent of children were born out of wedlock, the highest rate in the Western world.) Under this scenario, women choose multiple partners because they have more options available to them, they can rely on their support network during transitional times, and they have greater personal autonomy.
The second environmental context Scelza identified is where the sex ratio is female-biased (indicating a scarcity of men) or there is a high level of male unemployment (indicating a scarcity of men who can provide support). Women may be trying to “make the best of a bad situation and capitalizing on their youth to improve their reproductive prospects.” In such environments women tend to have higher rates of teen pregnancy as well as illegitimate births. Multiple mating may be a way of hedging their bets in an unstable environment. By pursuing an ardent sexual strategy, women are able to choose the best potential males as well as gain the support they need in order to maximize their reproductive success.
In many societies today, including our own, women who are overtly sexual and pursue multiple male partners often experience moral outrage and “slut shaming” of a kind that is entirely unheard of in other parts of the world. While these cultural attitudes used to look toward science for justification, that position is becoming increasingly difficult to reconcile with the biological evidence. From Sarah Hrdy’s discoveries among the langurs of Abu to polyamorous meetups in Aberdeen, female sexuality has been revealed to be a far more dynamic area of research than Darwin could have imagined. As Hrdy stated herself in the Annals of the New York Academy of Sciences at the dawn of the 21st century, far from being passive, females are “flexible and opportunistic individuals who confront recurring reproductive dilemmas and tradeoffs within a world of shifting options.” Or, as another observer summarized, “It’s our party. We can love who we want.”
This is the first in an ongoing series of columns that Eric Michael Johnson will be writing for Slate on the ways that evolution impacts our lives today.
Correction, Dec. 5, 2013: This article originally stated that saddle-backed tamarins are socially monogamous. Recent research has shown that they are not.