A new study from researchers at the University of New Mexico provides evidence that opposites do attract, Newsday reports. According to the study, women are more likely to be attracted to men who are genetically dissimilar to them, and more likely to be less sexually receptive and unfaithful to men whose genetic makeup is similar to their own.

These findings are based on the workings of a gene family called the major histocompatibility complex, or MHC, which helps the immune system recognize invaders. According to Newsday, the MHC is also responsible for how a person smells, and how other people will react to that smell. Researchers surveyed 48 couples that had been together, on average, for two years. They asked the women "a series of questions about their sexual responsiveness to their mate -- and their unfaithfulness in the relationship," Newsday summarizes. (The main question: "Does your partner turn you on?")

After the survey, the researchers compared the women's responses with the MHC data. The more similar the partners' MHC genes were, the less sexually responsive the women were to their partners, and the more they fantasized about other men prior to ovulation (previous studies have found that a woman's preference in sexual partners tends to change over the course of her menstrual cycle).

While early research suggested that male mice show preference for females with different MHC, the New Mexico study indicates that men do not exhibit a sexual preference based on MHC.

It's a little too soon to take the study's findings as gospel. For one thing, 48 couples is not that big a sample size. For another, the results rely on self-reported data. And measuring sexual responsiveness according to female fidelity is a pretty loaded concept.

Still, I think it's fascinating to see evolution in action. The results make sense: Genetically dissimilar people are more likely to shore up each other's genetic weaknesses. And I find researcher and professor Randy Thornhill's conclusion to be particularly poignant, although the Newsday article characterizes it as depressing. According to Thornhill, mate selection is a compromise between MHC attraction and other positive qualities, like doing the dishes or cracking great jokes. Once you find someone, he says, "love encourages you to do the best you can."