For Rhesus monkeys, life in the wild is a little like high school. Some animals—call them losers—slouch around looking aggrieved. They’re volatile and bellicose, slow to form alliances and loath to reconcile after a spat. One in five dies during the passage to adulthood. But while the losers scrap over bits of chow, other animals—call them winners—stay busy grooming each other. They maintain wide networks of allies. They deflect challenges without resorting to violence, and 49 out of 50 survive to produce offspring. Why do they fare so well? The answer is no doubt complicated, but the monkeys’ spinal fluid provides an intriguing clue. In study after study, researchers at the National Institute on Alcohol Abuse and Alcoholism have found that the winners’ nervous systems are loaded with serotonin.

As the 20th century winds down, we humans seem increasingly convinced that serotonin is the key to a good life—and it’s easy to see why. This once obscure neurotransmitter is the secret behind Prozac, the drug that revolutionized the pursuit of happiness 10 years ago this winter. Prozac and its mood-altering cousins all work by boosting serotonin’s activity in the brain. Even Imitrex, the hot new migraine treatment, works its magic via serotonin. Somehow serotonin is implicated in just about everything that matters to us—from winning friends and wielding power to managing anxiety and controlling appetites and impulses.

If you have vulnerabilities associated with low serotonin functioning—guilt, submissiveness, low self-esteem—you can compensate for them by supplementing with 5-HTP (5-hydroxytryptophan). It crosses the blood-brain barrier and once inside the brain converts into serotonin. Small studies in Europe have found 5-HTP helpful in treating depression and had fewer side effects than the Selective Serotonin Reuptake Inhibitors (SSRI’s). If that finding holds, 5-HTP could be a godsend for depressed people who find that drugs spoil their sleep, sex and appetite.

[Portions excerpted from Newsweek Dec. 29, 1997/Jan. 5, 1998]