Aug. 12, 2008 -- A woman is sexually attracted to men who smell like a good genetic match, but birth control pills make her desire the "wrong" men, a U.K. study shows.
Who is the right man? Studies suggest women are attracted to men whose genetic makeup differs from their own. Having a genetically different mate increases the chances for a healthy pregnancy and a healthy baby.
"If this really happens in the real world, women on the pill would end up choosing a more genetically similar mate than she would otherwise choose and the implications go on from there," study researcher S. Craig Roberts, PhD, tells WebMD.
Roberts notes that in an earlier U.S. study, women who were genetically similar to their partners reported being less satisfied in their sexual relationship with their partner -- and were seeking more new sex partners -- than were women with genetically dissimilar partners.
Animal studies show that female mammals can smell out males whose MHC genes are different from their own. MHC genes affect important immune responses. By mating with males who have different MHC genes, females give their offspring a better disease-fighting repertoire.
It's true of humans, too. In laboratory studies, women who sniff men's sweaty T-shirts find them more attractive when they come from men whose MHC genes don't match theirs. It's not that certain MHC genes smell better to women -- it's the difference that counts.
Rachel Herz, PhD, author of The Scent of Desire and a faculty member at Brown University, says there is a real connection between body odor, MHC, and the mates a woman chooses.
"My own research says the way a man smells to a woman is the main determinant of sexual attraction," Herz tells WebMD.
In earlier T-shirt-sniffing studies, women taking birth control pills seemed to be attracted to the "wrong" men. Intrigued, Roberts and colleagues took a closer look.
They paid 37 women to smell men's T-shirts before and after going on the pill. Then they compared the women's before- and after-pill ratings of the odors to those of 60 women who did not use oral contraceptives.
The result: After taking the pill, women shifted toward preferring genetically similar men. Women who did not take the pill slightly increased their preference for genetically different men.
Why? Roberts notes that when they become pregnant, female animals switch to preferring the scent of genetically similar males. This may allow them to seek out males that will help them protect and raise the baby. Claus Wedekind, PhD, who performed the original T-shirt-sniffing studies, has suggested that birth control pills somehow mimic this process.
The question, of course, is what happens when a woman taking birth control pills marries a man to whom she's attracted -- and then stops taking the pill.
Herz says marriage counselors who have never heard about these studies tell her that the No. 1 complaint among women no longer sexually interested in their husbands is that they can no longer stand how he smells.
"If you can't stand how someone smells, you cannot become intimate," Herz says.
Does this mean the birth control pill is a divorce pill? Herz says it's not that simple.
"A woman's response to a man's natural body odor will be colored by her feelings for him," she says. "So if you fell in love with a man online, it would be hard to be repelled by his smell."
Roberts and colleagues report their findings in the Proceedings of the Royal Society B: Biological Sciences.