A new evolutionary theory attempts to explain why about one in four human cultures performs a ritual: male genital cutting. Theories attempting to explain male genital cutting (also called male circumcision) have ranged from hygiene to sexual pleasure to an ancient desire to curb male sexual proclivities. University of New Mexico evolutionary anthropologist Gordon Gallup Jr., author of the "self-medication hypothesis" of the biological origins of drug abuse, and Jim Jordan of the University of New Mexico Medical School, propose in a new paper in the journal Evolution and Human Behavior that young boys' penises evolve more rapidly than the rest of their young bodies, making young boys more prone to disease and penile injury, potentially including death from infection. Boys whose parents cut off the foreskin of the penis might be more likely to reach adulthood and produce offspring. That would mean that genes that encode behaviors leading to penile cutting — the parental behaviors, not the behaviors of the boys themselves — might be selected over evolutionary time. In other words, the behavior, if true, would be maintained in a society via "kin selection," because it benefits siblings more than it does them. The Gallup and Jordan hypothesis also attempts to explain why men — and not women — are cut; why the procedure usually occurs among young boys rather than adults; why other body parts aren't cut; why the practice may offer no discernible benefit to health or hygiene; why it varies so widely across cultures and epochs; why it continues even where there's no risk of HIV or other sexually transmitted diseases; and why it's often couched in terms of religion, social bonding or tribal identity. "Why don't we see people cutting off fingers, toes, and so on?" Gallup asked in an interview. "They're just focusing on the penis. Why? This is all consistent with the idea that a young boy's penis, evolutionarily, is vulnerable to disease and injury, particularly as penises enlarge from babyhood. I just don't see any other theory that can explain everything." The Gallup-Jordan paper cites anthropological, ethnographic and other data from various sources, including the World Health Organization, the demographic website Popline and the World Culture Encyclopedia, to support the kin-selection hypothesis. Gallup has researched other theories, including those related to hygiene and sexual pleasure, and finds them wanting or disproven. The two researchers acknowledge the limitations of their theory and the difficulty in studying the practice because of the "understandably sensitive" nature of the subject. There is no worldwide database on male genital cutting, though the United Nations Children's Fund (UNICEF) reported in 2005 that 30 percent of men worldwide are cut. The U.S. Centers for Disease Control and Prevention reported in 2010 that 56.4 percent of boys born in the United States in 2009 had undergone circumcision. Some cultures, such as many Muslim and North African cultures, circumcise boys relatively soon after birth. Others, such as the Jews and some Australian Aboriginals, cut the foreskin when the boy is older, usually before adulthood. Male circumcision is rare or completely absent in most of Europe, Scandinavia, East Asia, South Asia and Latin America. One critique is that the kin-selection hypothesis is difficult to test, said Peter Ellison, an anthropologist at Harvard University. Also, Ellison suggested that the benefits of male genital cutting should also apply to girls. "If cutting is to prevent infectious disease and injury, then logically a similar intervention would be of equal or greater utility in females," he said by email. Other critics contend that the theory of kin selection is simply a warmed-over version of an old theory that links male genital cutting to mothers' postpartum depression and the related theory that penile cutting is practiced in places where it benefits women at the expense of men. Gallup and Jordan, however, are unapologetic about the implications of their hypothesis, which suggest that the practice is simply an unfortunate consequence of human evolution. "You can say it's a bad idea, we shouldn't do this, but my point is, evolution doesn't care about that," Gallup said. "In this environment, boys with cut penises are more likely to survive. It isn't so much a matter of male sexual promiscuity, or trying to control women by cutting men, or of women trying to control men by pressuring them to cut their sons. It seems to be purely a consequence of evolution."