How do I love thee? Let a poet count the ways. I love you to the depth and breadth and height my soul can reach, to the level of everyday's most quiet need; freely, purely, with the passion of old griefs and childhood's faith, with a love lost with lost saints, with the breadth, smiles, tears, of all my life. Why do I love thee? Let anthropologist Helen Fisher count the ways. I love you because of a complex chemistry of dopamine, oxytocin, vasopressin, testosterone and norepinephrine; because I have a large caudate nucleus, amygdala, and hippocampus; because I walk upright and have helpless babies and I need you to protect me from predators. I love you because your testosterone has made you taller and stronger than me and given your jaw a handsome square cut.

Not very romantic, but what do you expect from a physiologic, evolutionary, anthropological assessment of love? For Fisher is a love researcher. That's right. Love researcher. And she has collected the results of this research in her new book Why We Love : The Nature and Chemistry of Romantic Love.

And it's quite a collection. For example, did you know that there is a natural anti-depressant in seminal fluid? That there is a National Broken Hearts Day? That love's passions are the same across such disparate cultures as the United States and Japan? OK, maybe you knew that, since poetry across all cultures has so much to say on the topic. Did you know that love is an addiction? Of course you did.

But, did you know that the biochemistry of love is the same as the biochemistry of addiction? That's what Fisher's research shows. She has scanned the brains of people who are newly, deeply, and passionately in love and found that they have increased activity in areas that (surprise) are associated with arousal and concentration, such as the caudate nucleus and the ventral tegmental area, the bottom portion of the brain that is high in dopamine producing cells. It is dopamine that's responsible for the cocaine addict's high, the chocolate lover's satisfaction, the cigarette smoker's contentment, and, evidently, the lover's passionate obsession. Fisher has also scanned the brains of dejected and rejected lovers, and although she doesn't share those results in the book, her thesis is that the dejected suffer from a depletion of dopamine, leading to feelings very similar to withdrawal.