If you're a woman who likes sleeping in a significant other's T-shirt, there's a reason for that. Rather than kissing a lot of frogs, scientists say that women have to sniff a lot of men to find their perfect match. What does the nose know? It may be able to smell men with completely different gene pools.
In several studies, researchers have had women smell men's used T-shirts and rank them according to how attractive the smell is. In the past, women have indicated that the most pleasurable shirts belong to men with different major histocompatibility complex (MHC) genes than they do, so scientists believe that women can subconsciously smell a man's genes [source: Kaplan].
MHC genes, which affect the immune system, have been determined to play a role in everything from sexual attraction to marital happiness. For example, in 2007, Christine Garvar-Apgar of the University of New Mexico conducted a study of the satisfaction in 48 heterosexual marriages. She found that women who had vastly different MHC genes from their husband's were more likely to have happy marriages and to report fidelity in their relationships. However, women who had similar MHC genes were more likely to be less sexually responsive to their partners and more likely to have had or considered an affair [source: Kaplan]. These adulterous longings even seemed to sync up with MHC correlation: If the couple had 50 percent of the MHC genes in common, the woman had a 50 percent chance of cheating [source: Kaplan]. MHC genes played no role in a man's happiness or wandering eye.
Women may be seeking out opposite gene pools in order to give their unborn children the best immunological head start possible. Since MHC genes play such a large role in immune function, a child's body with many different MHC genes will have a greater ability to detect and ward off invading cell bodies that may carry disease.
However, when women take birth control pills, it can affect their sense of smell. In a 2008 study, Stewart Craig Roberts of the University of Newcastle found that women on the pill tended to select mates that had similar MHC genes [source: Bryner]. This may be because birth control tricks a woman's body into thinking that it's pregnant, so that a woman is subconsciously sniffing for a relative that will help her care for her nonexistent baby. Researchers hypothesized that women could meet, fall in love and marry a man, only to have the relationship crumble once they went off birth control and got a whiff of the guy's true smell.
Still looking for your opposite gene pool? Check out more articles on love on the next page.