NATALIE ANGIER - ESSAY

In the United States and much of the Western world, when a couple divorces, the average income of the woman and her dependent children often plunges by 20 percent or more. The income of her now unfettered ex - who had been the family’s primary breadwinner but who rarely ends up paying in child support what he had contributed to the household finances - climbs accordingly. The born-again bachelor is therefore perfectly positioned to attract a new, younger wife and begin building another family.

Small wonder that many Darwinian- minded observers of human mating customs have long contended that serial monogamy is just a socially sanctioned version of harembuilding. Women, by contrast, are not thought to be natural serializers. Sure, a girl might date around when young, but once she starts a family, she is assumed to crave stability.

Yet in a report published in the summer issue of the journal Human Nature, Monique Borgerhoff Mulder of the University of California, Davis, presents compelling evidence that at least in some non-Western cultures where conditions are harsh and mothers must fight to keep their children alive, serial monogamy is by no means a man’s game.

To the contrary, Dr. Borgerhoff Mulder said, among the Pimbwe people of Tanzania, serial monogamy looks less like polygyny than like a strategic beast that some evolutionary psychologists dismiss as quasifantastical: polyandry, one woman making the most of multiple mates.

In her analysis, she found that although Pimbwe men were somewhat more likely than their female counterparts to marry multiple times, women held their own and even outshone men in the upper end of the scale, of five consecutive spouses and counting. And when she looked at who extracted the greatest reproductive payoff from serial monogamy, as measured by who had the most children survive past the first five hazardous years of life, she found a small but significant advantage for females. Women who worked their way through more than two husbands had, on average, a greater number of surviving children than the more sedately mating women.

Among the women, those with the greatest number of spouses were themselves considered high-quality mates, the hardest working, the most reliable, with scant taste for alcohol. Among the men, by contrast, the higher the nuptial count the lower the ranking, and the likelier the men were to be layabout drunks.

“We’re so wedded to the model that men will benefit from multiple marriages and women won’t, that women are victims of the game,” Dr. Borgerhoff Mulder said. “But what my data suggest is that Pimbwe women are strategically choosing men, abandoning men and remarrying men as their economic situation goes up and down.”

The new analysis, though preliminary, is derived from one of the more comprehensive data sets yet gathered of marriage and reproduction patterns in a non-Western culture. The results underscore the importance of avoiding the generalities of what might be called Evolution Lite, an enterprise too often devoted to proclaiming universal truths about deep human nature. Throughout history and across cultures, Dr. Borgerhoff Mulder said, “there has been fantastic variability in women’s reproductive strategies.”

The Pimbwe live in small villages and eke out a subsistence living farming, fishing, hunting and gathering. There is little formal sexual division of labor. “In terms of farming, men and women do pretty much the same tasks,” Dr. Borgerhoff Mulder said. “The men will cook, do a lot with the kids.”

Unlike in the West, where men control a far greater share of resources than women do, or in traditional pastoral societies like those found in the Middle East and Africa, where a woman is entirely dependent on the wealth of her husband, Pimbwe women are independent operators and resourceful co-equals with men.

This does not mean that mothers can go it alone, however. Again in contrast to the contemporary West, childhood mortality remains a serious threat, and it takes the efforts of more than one adult to keep a baby alive. A good, hardworking husband can be a great asset - and so, too, may his relations.

The evolutionary theorist Sarah Blaffer Hrdy proposes that one reason the offspring of much-marrying Pimbwe women do comparatively well is that the children end up with a widened circle of caretakers. “The women are lining up more protection, more investment, more social relationships for their children to exploit,” she said. “A lot of what some people would call promiscuous I would call being assiduously maternal.”