Helen Fisher, an anthropology professor at Rutgers University, along with two colleagues, used an MRI machine to study the brains of people who described themselves as being wildly in love. When each subject gazed at a photograph of his or her sweetheart, the ventral tegmental area and the caudate nucleus in the brain lit up. The caudate nucleus is the site of a dense network of receptors for the neurotransmitter dopamine. Dopamine neurons are activated when an unexpected reward is presented.

Donatella Marazziti, a psychiatry professor at the University of Pisa in Italy, also measured the levels of neurotransmitter serotonin in the blood of people who'd been in love for several months and who pined for their lovers for at least four hours a day. She found that the serotonin levels in the lovesick subjects were as low as the serotonin levels in people with obsessive-compulsive disorder (OCD). So if you're feeling "madly" in love, there's a solid scientific basis for losing both your heart AND your mind.