Let me be clear: I still love my husband. There is no man I desire more. But it's hard to sustain romance in the crumb-filled quotidian that has become our lives. The ties that bind have been frayed by money and mortgages and children, those little imps who somehow manage to tighten the knot while weakening its actual fibers. Benjamin and I have no time for chilled white wine and salmon. The baths in our house always include Big Bird.

If this all sounds miserable, it isn't. My marriage is like a piece of comfortable clothing; even the arguments have a feel of fuzziness to them, something so familiar it can only be called home. And yet . . .

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.