A woman enters a room alone and seats herself before a table covered with seven small boxes. She picks up each box, sniffs it carefully, and jots something on a pad of paper. Shortly after she leaves, another woman enters and reenacts the performance. He has found that women have strong preferences in that department. They're attracted to the scent of men who are most unlike them in a very particular way in the array of immune system genes known as MHC, for major histocompatibility complex.
MHC genes are the most diverse of all genes.
In fact, they differ so widely from person to person that they constitute a molecular John Hancock, one that helps an organism recognize its own healthy cells, identify pathogens, and reject foreign tissue. The idea that they may also confer a distinctive odor, and thereby influence behavior, was first suggested in 1974 by biologist Lewis Thomas. Laboratory studies soon proved Thomas right in the case of mice. Moreover, their odor preferences were not innate but learned. Since the mouse studies were done, researchers have found that humans, too, can detect MHC-dependent odors.