Summary: the pill gives women a sexual preference for men they won’t want to stay with later. They go off the pill, have kids, then discover they can’t stand their partners.

Major histocompatibility complex (MHC) genes are involved in immune response and other functions, and the best mates are those that have different MHC smells than you. The new study reveals, however, that when women are on the pill they prefer guys with matching MHC odors.

MHC genes churn out substances that tell the body whether a cell is a native or an invader. When individuals with different MHC genes mate, their offspring’s immune systems can recognize a broader range of foreign cells, making them more fit.

Past studies have suggested couples with dissimilar MHC genes are more satisfied and more likely to be faithful to a mate. And the opposite is also true with matchng-MHC couples showing less satisfaction and more wandering eyes.

“Not only could MHC-similarity in couples lead to fertility problems,” said lead researcher Stewart Craig Roberts, an evolutionary psychologist at the University of Newcastle in England, “but it could ultimately lead to the breakdown of relationships when women stop using the contraceptive pill, as odor perception plays a significant role in maintaining attraction to partners.”

Sexy scents

The study involved about 100 women, aged 18 to 35, who chose which of six male body-odor samples they preferred. They were tested at the start of the study when none of the participants were taking contraceptive pills and three months later after 40 of the women had started taking the pill more than two months prior.

For the non-pill users, results didn’t show a significant preference for similar or dissimilar MHC odors. When women started taking birth control, their odor preferences changed. These women were much more likely than non-pill users to prefer MHC-similar odors.

“The results showed that the preferences of women who began using the contraceptive pill shifted towards men with genetically similar odors,” Roberts said.

http://news.yahoo.com/s/livescience/20080813/sc_livescience/thepillmakeswomenpickbadmates

How bad is it? Well… I think when the lawsuits start, it’s going to be the biggest bloodletting in recent legal history.