Almost 25 percent of indigenous societies practice some form of male genital cutting, ranging from circumcision to the ritual removal of a testicle. The reason, reports a Cornell scientist, may be to reduce pregnancies from extramarital sex. More subtly, it could be to reduce conflict among men.

Chris Wilson, a doctoral candidate in Cornell's Department of Neurobiology and Behavior, observes that cultures often express the reasons for what anthropologists call "male genital mutilation," in terms that have immediate meaning, such as religion, tradition, hygiene or initiation into adulthood. The new research suggests that these psychological rationales exist to serve a deeper evolutionary purpose in certain societies, even though men are not consciously aware of the complex evolutionary logic shaping their thoughts and behavior.

Writing in the journal Evolution and Human Behavior, Wilson says that because genital alteration is a painful and even risky procedure, especially under primitive conditions, it must have some evolutionary benefit or it would not have persisted.

Wilson hopes that the evolutionary explanation he offers for genital cutting will prove useful to anthropologists, doctors and policy-makers as they grapple with cultural, ethical and medical issues surrounding the ancient practice. The evolutionary origin of circumcision may be of especially broad interest, as this particular operation is not only performed in 20 percent of indigenous societies, but on approximately one-third of all men worldwide.

In his research article, Wilson asks, "Why have 180 cultures all converged on this practice?" The different types of cutting suggest that several societies independently developed the practice before recorded history.

Wilson, who works with Paul Sherman, Cornell professor of neurobiology and behavior, says that genital cutting may limit extramarital sex. In evolutionary terms, a man benefits from such affairs by passing his genes to a child who requires no further investment on his part.

"If natural selection has designed the genitals for fertilization, then changing that design will harm this function," says Wilson. "In particular, modifying the shape of the genitals makes it physically less likely that a man will impregnate a woman during an affair, and, therefore, the evolutionary incentive for adultery is smaller."

He explains that the procedure therefore allows men within a society to trust each other more, because it reduces conflict over paternity and sexual indiscretion. In the indigenous societies that practice genital cutting, Wilson suggests that the social benefits outweigh the costs, and so the custom persists.

His research found much higher rates of cutting in societies where men have multiple wives, especially when wives live far apart. In these cultures, the opportunities for extramarital affairs are high because a husband can't keep a close eye on all of his wives at once. Genital alteration acts as a physical signal of sexual honesty, reducing mistrust between the married and unmarried men.

After controlling for numbers of wives, Wilson also found lower rates of extramarital affairs in societies that practice male genital cutting, compared with those that do not, suggesting that it does indeed play a role in limiting adultery.

Finally, among societies practicing genital alteration, older men gave trust and benefits to younger men who underwent the procedure, supporting Wilson's theory that the procedure improves trust and social status.

Amelia Apfel '08 is a writer intern at the Cornell Chronicle.