Rhesus monkey, only 4 months old, has been taken from the shoulders of its mother and put into a small steel cage. Within minutes the fright in its little brown eyes softens into curiosity, and stepping carefully, the monkey makes its way across the cage. But when a man with a white hood over his head walks in and stops two feet away, the baby freezes. It pulls its body away from the cage and quietly looks the other way, appearing to ignore the man until, two minutes later, be leaves."This is a very inhibited response," says Dr. Judy Cameron, a neurobiologist and the director of the University of Pittsburgh’s primate laboratory, watching the monkey through an opening in a curtain. "Obviously, the intruder caused this animal a great deal of anxiety."More important, for Dr. Cameron’s purposes, the baby monkey can be compared to a highly anxious or depressed human child. Now researchers are putting a spotlight on teenage depression, hoping that by learning how the disorder takes root, they can find ways to keep it from growing into a weak condition and lifelong problem for 9.5 percent of adults.The monkey studies, it is hoped, will add to the effort to make clear which aspects of brain function are involved in adolescent depression. In monkeys, depression is difficult to diagnose because, obviously, the animals cannot talk about feelings. Anxiety, though, is fairly easy to measure by observing behavior."Now, we can do all sorts of studies with the anxious monkeys that are harder to do in humans," Dr. Cameron said. "We can observe their behavior every day. We can take blood samples and look at their hormones. We can assess how they respond to stress. We can ask what’s different about their brains. And ultimately, we can search for genes that may explain people’s anxiety." Within minutes after the little monkey is put into a small steel cage, the fright in its little brown eyes softens into().