Researchers report genetic evidence bolstering the socially contentious idea that polygyny—the mating practice where some males dominate reproduction by fathering children with several women—was the norm for sexual behavior throughout human history and prehistory. Because polygyny means other men father few or no children, the study, published today in PLoS Genetics, also shows that, on average, women bequeath more genes to their offspring than men do. 

The proportion of female to male genes passed on is not yet known. "Our follow-up work is to get a better estimate, but we believe it's at least two to one, if not more," says senior study author Michael Hammer, a geneticist at the University of Arizona in Tucson.

"This is good science, and even more notable is the increasing light it sheds on our own human nature," says David Barash, evolutionary psychologist, University of Washington in Seattle.

The study, which examined genetic material (DNA) from six geographically diverse populations—Biaka from Central African Republic, Mandenka from Senegal, San from Namibia, French Basque, Han Chinese and Melanesians from Papua New Guinea—provides independent corroboration of what many animal studies have shown and evolutionary biologists have long claimed: basic human biology is polygynous, Barash notes. "Monogamy is a recently inspired cultural add-on."

Researchers examined DNA areas devoid of genes in each of 90 people, including 20 regions on the X chromosomes (present in both male and female mammals) and 20 on the autosomes (the other 22 chromosomes, which are not involved in sex determination). If an equal number of males and females breed successfully, genetic variation in the two kinds of chromosomes should be about equal. Instead, the researchers found much more variation on the X than on autosomes.

The paper explored explanations for this X genetic diversity, concluding that polygyny was the most likely.

The notion that monogamy is not "natural" for humans has been controversial because some fear it may provide a biological justification for promiscuity. "But it's a fallacious idea that we can infer from what is the case something about how we ought to act," says Erik Parens, senior research scholar at The Hastings Center bioethics think tank based in Garrison, N.Y. "I don't see why we should accept the premise that we can read off of how our forebears acted…[the way]…we ought to act now."