Will your mate cheat on you?
That depends on whether the two of you share a particular set of genes known as MHC. Research released in the October issue of Psychological Science suggests that the likelihood your mate will cheat increases with greater genetic similarity in a certain region of chromosome 6. Women, but not men, whose partner's MHC matches their own tend to find their partner less sexually interesting, especially when ovulating. They are also more strongly attracted to other men.
MHC codes for chemical markers on the surface of immune cells that help catch foreign cells - those that could potentially cause illness or disease. The more kinds of MHC genes a person has, the more germs he or she will be able to fend off. Individuals who have similar MHC genes are more likely to produce a child whose immune system can't detect and fend off as many kinds of germs.
When women are paired up with a men whose immune system genes are similar to their own, they tend to find other men more sexually appealing about the time they are likely to become pregnant. This appears to be a behavioral mechanism to avoid inbreeding.
So, why don't men show a similar effect? The authors of the paper don't get around to talking about this, but the glib answer would be that men are attracted to women other than their mates already anyway. They don't need any extra help from an inbreeding avoidance mechanism to spread their seed when it's so cheap, so to speak.
A woman's egg is comparatively rare and expensive, so the onus is on her to choose a mate wisely. This view suggests the smart choice is on someone who is genetically very different. That person isn't always the one who turns out to be a reliable life partner for raising children cooperatively. Other research has found that men with diverse MHC genes tend to be symmetrical, physically attractive, and also promiscuous. They tend to be cads rather than dads.
Cheating may present a way for women to have their cake and eat it too. By securing a dependable but less sexy partner, then cheating on him and becoming pregnant by a genetically better man, a woman can secure for her baby the good genes for a healthy immune system *and* a reliable dad. That is, if the cuckolded dad never figures it out.
On average, 1 in 10-25 children are born this way. It's no surprise then that a study published in Nature a few years ago found at that maternal relatives are more likely to say that a newborn looks like her dad. It's in their interest to try to convince the dad that the child really is his.