A few years ago, Yeshiva University neuroscientist Lucy Brown and her research team distributed flyers across several campuses in the New York area to recruit participants for a brain-imaging study. The flyers had one sentence highlighted: "Have you just been rejected in love but can't let go?" Soon enough, Brown recalls, she had college students—who were asked to bring a photo of their beloved with them—crying in the brain scanner.

The brains of the forlorn study subjects looked a lot like drug addicts looking for a fix. Brown concluded, "In retrospect, it's not surprising that the same areas of the brain that were active in the brains of cocaine addicts were active in these people who were heartbroken looking at a picture of their former romantic partner." "We crave the other person just as we crave nicotine or pain pills; you want to be near the other person, you're constantly thinking about them, we even do dangerous things sometimes to win them back—we don't eat or sleep."