The Divorce Pill?

Women generally prefer the smell of men whose MHC gene complements are different from theirs, setting the stage for the best biological match. But Wedekind’s T-shirt study revealed one notable exception to this rule: women on the birth-control pill. When the pill users among his subjects sniffed the array of pre-worn T-shirts, they preferred the scent of men whose MHC profiles were similar to theirs—the opposite of their pill-free counterparts.

This dramatic reversal of smell preferences may reflect the pill’s mechanism of action: It prevents the ovaries from releasing an egg, fooling the body into thinking it’s pregnant. And since pregnancy is such a vulnerable state, it seems to activate a preference for kin, who are genetically similar to us and likely to serve as protectors. “When pregnant rodent females are exposed to strange males, they can spontaneously abort,” Herz says. “The same may be true for human females.” What’s more, some women report a deficit in sex drive when they take the pill, a possible consequence of its pregnancy-mimicking function.

The tendency to favor mates with similar MHC genes could potentially hamper the durability of pill users’ relationships in the long term.

i.e. the pill is destroying an important evolutionary mate selection strategy which maintains the strength of the human immune system. Oops. We really need to upgrade birth control technology.

http://www.psychologytoday.com/articles/index.php?term=pto-20071228-000001.xml&page=3