Relationship quizzes in magazines are fun, but a test for genetic compatibility might be the better way to go to see if your wife or girlfriend will cheat on you.

A new study reveals that a cluster of genes, involved in immune function among other things, could predict how sexually attracted a person is to a partner and how likely a woman is be faithful to her mate. Couples in which the individuals had dissimilar versions of so-called major histocompatibility complex (MHC) genes had the greatest sexual compatibility.?

"There's this idea of romantic chemistry, but until now we haven't been able to pinpoint anything that predicts it," said lead researcher Christine Garver-Apgar, a psychologist at the University of New Mexico.

"These are some of the first findings that I know of that get at this idea of romantic chemistry and what it is exactly that makes two people just so compatible and attracted to each other," said Garver-Apgar, whose husband is related to Virginia Apgar, who developed in 1952 the Apgar score used today in hospitals to rate a newborn's health.

The MHC genes direct the production of certain protein receptors that coat the outer surfaces of cells. The protein receptors signal to the body's immune system whether a cell is a native resident or a foreign invader. With more MHC variations, the immune system can recognize a broader range of foreign cells, making associated offspring more fit.

All you need is DNA ...
Garver-Apgar and her colleagues studied 48 romantically involved couples, ranging from 18 to 35 years old. For genetic material, they scraped cells from the inner cheeks of subjects.

The couples completed surveys at the start of the study, when the female partner was at the fertile part of her menstrual cycle and during her infertile period. Questions gauged a person's overall satisfaction with the current romantic relationship, contentment with in-couple sex, number of sex partners and attitude toward sex in general.

A measure of female subjects' luteinizing hormone, which regulates egg production, indicated the stage of menstrual cycle and level of fertility.

It turned out that opposites did attract, particularly when women were most fertile. "As the [MHC] similarity increases, women are more turned off toward the guy sexually and more likely to be fantasizing about other men, specifically when she's at the fertile point in her cycle," said study team member Randy Thornhill, a biologist at the University of New Mexico.

Not only did they fantasize, but women in similar-MHC relationships reported more sexual encounters outside with other men.

"This speaks to the possibility that women do seek sex outside of the relationship for a particular reason and it's to possibly obtain genetic benefits, whether those are good genes or compatible genes," Garver-Apgar told LiveScience.

Men showed no discrimination when it came to sexual desire toward their partners. That supports the idea that men don't put as much energy into reproduction. They just copulate when the opportunity arises.

Sexual chemistry
Humans might be able to sniff out a mate's genes to discern this immunological compatibility. Past studies have shown that some species of mice, birds, fish and lizards can identify a potential mate's MHC type through smell.

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Somehow MHC genes could set in motion the formation and release of scent molecules through a person's glands on their skin, the researchers suggest in the October issue of the journal Psychological Science.

Whether an MHC test could truly be more predictive of mate compatibility than magazine quizzes is outside of scientists' realm. But cheek-scraping for DNA material is simple.

"All you need is some DNA," Thornhill said, "because the DNA is going to contain the chemistry of that person's genes."