One common complaint Dr. Berman hears among women with relationship problems is that they love their partner, but they're not in love with him. "There is a difference between love and chemistry," Dr. Berman says. "Take a really good whiff of him without cologne when he's relatively clean. If that smell turns you on, that's a really good cue of chemistry."

In one recent study, Dr. Berman says researchers had women smell men's T-shirts. The women were most attracted to the shirts of men with a different major histocompatability complex (MHC) from them. MHC is a collection of genes that are related to immune systems. "We unconsciously want to mate with someone who has a different immune system than ours because that helps with the survival of our offspring," Dr. Berman says.