Don't expect defense lawyers for Dominique Strauss-Kahn (or divorce lawyers for Arnold Schwarzenegger) to use it as evidence to explain their client's marital infidelities. Still, there's no questioning the relevance these days to a new study that finds the genetic make-up of an individual -- male or female -- may help explain a tendency to adopt sexually unfaithful behavior.

Researchers at the Max Planck Institute For Ornithology in Seewiesen, Germany, have found evidence of genetic-driven infidelity among zebra finch birds. And in their findings, published this week in the prestigious PNAS scientific journal, the researchers do not hesitate to draw similar conclusions with human beings.

Of course, there are many other socio-cultural factors that more or less condition the sexual behavior of homo sapiens: religious rules, social taboos, chance encounters. But there is nothing precluding the possibility of a genetic influence as well. Conducting similar studies on humans in order to verify such findings is nearly impossible, and very difficult for other mammals -- just 10% of all species are monogamous.

Researcher Wolfgang Forstmeier and his colleagues focused their study on zebra finches, a bird species known to be faithful to their partners, but also with a noted ability to breed easily. And they focused on the behavior of females more than of males.

It is recognized that, from an evolutionary perspective, the goal of an individual's life is "to maximize one's genetic patrimony," says the Swiss primatologist Jörg Hess. In practice, this means that the male member of a species will attempt to mate with as many females as possible in order to produce a maximum number of descendants. Females don't have that same incentive.

Why do females take the risk?

"There is much research focused on understanding why certain females, even though they have already mated with a male, would still risk mating with another male," says Forstmeier. "This promiscuity has no clear advantage for them, and in fact could come at a great cost (transmission of diseases, the male partner no longer caring for the offspring)."

In order to see things more clearly, the biologists used 1,500 finches of five consecutive generations. They placed in cages several finches, male and female, whose father was known to be very or somewhat unfaithful. With a video camera, they observed the finches mating. Above all else, they managed to quantify, over several generations, the propensity of each finch to engage in "extramarital" relations.

The result? "We discovered that infidelity is correlated between males and females, in other words, that the sister of an unfaithful male is more likely to have a penchant for infidelity as well. In effect, the two birds share the same genetic, inherited predisposition of their fickle ancestors."

The researcher admits that "even among birds, the attitude of the female does not entirely depend on her genetic make-up, but probably for at least half. Even so, the correlation is big enough to be significant. That would explain why, even if she does not garner any direct benefits, the female still leans ‘to the left," as her genes ‘prompt" her to do so."

For Laurent Keller, a biologist at the Department of Ecology and Evolution at the University of Lausanne, "these results are not surprising, as all personality traits are defined, at least in part, by one's genetic make-up. Furthermore, when the environmental conditions are similar for all of the individuals of several generations, as was the case for these birds raised in captivity, the inheritability of a trait becomes stronger than in the wild." Keller says that better conclusions will be possible once these results are compared with similar data gathered from birds taken at random from nature.

What also remains to be explained is how exactly genes "push" an individual into having an extramarital affair. "It is nearly impossible," says Forstmeier, "because of an infinite complexity of interactions at various levels of a personality (exclusive social ties, sexual motivation, acceptance of risk)."

Other researchers, however, have attempted to do just that. In 2008, Hasse Walum of the Karolinska Institute in Sweden screened 900 men who had been in a relationship for five years for vasopressine, a hormone tied to social behavior. Those who had a mutated form of the gene had rockier relationships, and were less attached to their partner.

This study is itself based on even more remarkable work conducted in 2004. Larry Young of Emory University in Atlanta used two species of vole (a small rodent), the prairie vole (microtus ochrogaster), social and monogamous, and the meadow vole (microtus pennsylvanicus), asocial but polygamous. He noticed a difference in their brains. In the monogamous rodent, vasopressine receptors were situated in the same area as dopamine, the pleasure hormone secreted during sexual intercourse. Among the meadow vole, aka "mustachioed Don Juan," the two receptors were far from each other, so much that the vole establishes no relation between its pleasure and a particular individual.

Read the original article in French

photo - robad0b