So what, really, is this thing called love?

According to the new study scientists say that the brain chemistry of infatuation is akin to mental illness—which gives new meaning to "madly in love."

In the Western world we have for centuries concocted poems and stories and plays about the cycles of love, the way it morphs and changes over time, the way passion grabs us by our flung-back throats and then leaves us for something saner. If Dracula—the frail woman, the sensuality of submission—reflects how we understand the passion of early romance, the Flintstones reflects our experiences of long-term love: All is gravel and somewhat silly, the song so familiar you can't stop singing it, and when you do, the emptiness is almost unbearable.

We have relied on stories to explain the complexities of love, tales of jealous gods and arrows. Now, however, these stories—so much a part of every civilization—may be changing as science steps in to explain what we have always felt to be myth, to be magic. For the first time, new research has begun to illuminate where love lies in the brain, the particulars of its chemical components.

Anthropologist Helen Fisher may be the closest we've ever come to having a doyenne of desire. At 60 she exudes a sexy confidence, with corn-colored hair, soft as floss, and a willowy build. A professor at Rutgers University, she lives in New York City, her book-lined apartment near Central Park, with its green trees fluffed out in the summer season, its paths crowded with couples holding hands.

Fisher has devoted much of her career to studying the biochemical pathways of love in all its manifestations: lust, romance, attachment, the way they wax and wane. One leg casually crossed over the other, ice clinking in her glass, she speaks with appealing frankness, discussing the ups and downs of love the way most people talk about real estate. "A woman unconsciously uses orgasms as a way of deciding whether or not a man is good for her. If he's impatient and rough, and she doesn't have the orgasm, she may instinctively feel he's less likely to be a good husband and father. Scientists think the fickle female orgasm may have evolved to help women distinguish Mr. Right from Mr. Wrong."

One of Fisher's central pursuits in the past decade has been looking at love, quite literally, with the aid of an MRI machine. Fisher and her colleagues Arthur Aron and Lucy Brown recruited subjects who had been "madly in love" for an average of seven months. Once inside the MRI machine, subjects were shown two photographs, one neutral, the other of their loved one.

What Fisher saw fascinated her. When each subject looked at his or her loved one, the parts of the brain linked to reward and pleasure—the ventral tegmental area and the caudate nucleus—lit up. What excited Fisher most was not so much finding a location, an address, for love as tracing its specific chemical pathways. Love lights up the caudate nucleus because it is home to a dense spread of receptors for a neurotransmitter called dopamine, which Fisher came to think of as part of our own endogenous love potion. In the right proportions, dopamine creates intense energy, exhilaration, focused attention, and motivation to win rewards. It is why, when you are newly in love, you can stay up all night, watch the sun rise, run a race, ski fast down a slope ordinarily too steep for your skill. Love makes you bold, makes you bright, makes you run real risks, which you sometimes survive, and sometimes you don't.