Maybe it's not similar interests, horoscope signs, looks, or proximity that make women and men fall in love. According to evolutionary scientists, when people throw up their hands and say "it was just chemistry," they may be on to a fundamental factor in mate choice.

Subtle chemical signals, or pheromones, have long been known to draw pairs together within the same species, and for a specific reason. In mice, for example, experiments showed that pheromones acted as attractants between males and females who were genetically similar except that they differed in a certain type of immune system gene. That difference is actually a survival benefit: The combination of two individuals' different MHC (major histocompatibility locus) genes gives their offspring an advantage in beating back disease organisms.

So the mice could smell a genetic difference. But could modern humans, who aren't known for a particularly good sense of smell, also make that distinction?

In the first "sweaty T-shirt" experiment, a Swiss zoologist, Claus Wedekind, set up a test of women's sensitivity to male odors. He assembled volunteers, 49 women and 44 men selected for their variety of MHC gene types. He gave the men clean T-shirts to wear for two nights and then return to the scientists.

In the laboratory, the researchers put each T-shirt in a box equipped with a smelling hole and invited the women volunteers to come in, one at a time, and sniff the boxes. Their task was to sample the odor of seven boxes and describe each odor as to intensity, pleasantness, and sexiness.

The results were striking. Overall, the women preferred the scents of T-shirts worn by men whose MHC genes were different from their own.

The experiment did not test men's perceptions of female scents, but the results certainly suggest that evolution has provided humans, not just mice, with a transmitter and receiver for genetic information that could influence mate choice.

And all this even before the first date!