I also examined an alternative hypothesis suggesting that MGM signals group commitment for collective action, particularly inter-societal warfare.
Although other forms of male scarification fit this model, the distribution of MGM is not predicted by frequency of inter-societal warfare. put forward a theory that the shape of the glans has evolved with the function of pumping a rival's sperm out of the vagina, tending to ensure that a child born after that intercourse is that of the man concerned and not an earlier one.
So why do some societies insist on such a risky ritual for their men?
There may be an evolutionary explanation, according to Christopher Wilson, of Cornell University in New York, US.
Their widely-quoted study does not mention the foreskin.
One of the popularisers of that theory, Jesse Bering, put readers' questions to Gallup for Scientific American (May 30, 2009) about that issue.
READERS: The latex genitalia study wasn't terribly convincing because the models were circumcised, and in real life the foreskin would interfere with the semen-displacing functions of the coronal ridge.
Sperm competition theory predicts that males will evolve ways to ensure that their sperm, and not another males, fertilises a females eggs.
Men who display this signal of sexual obedience may gain social benefits if married men are selected to offer social trust and investment preferentially to peers who are less threatening to their paternity.
Clitoridectomy and vaginal infibulation serve a parallel signaling function in women, increasing a husband's paternity certainty and garnering his increased investment.
I suggest that MGM is likely to reduce insemination efficiency, reducing a man's capacity for extra-pair fertilizations by impairing sperm competition.
MGM may therefore represent a hard-to-fake signal of a man's reduced ability to challenge the paternity of older men who are already married.
But if a man with, say, four wives wants to ensure that any children his wives produce are his, there is pressure to make sure other men cant successfully impregnate them. If the sperm competition theory is correct, he reasoned, then male genital mutilation should be more common in societies where men tend to have multiple wives, especially those in which the wives live apart from the husband. Wilson searched anthropological databases and found that his predictions were borne out: 48% of highly polygynous societies practice some form of male genital mutilation, and in societies in which wives live in separate households that increases to 63%.