To make the anxiety-provoking lab challenge more authentic and emotionally charged, Pine and his colleagues at the N.I.M.H., Eric Nelson and Amanda Guyer, concocted an elaborate experimental setup to persuade teenagers in a functional M.R.I. machine that their social status really is on the line: a fake Internet chat room. They created a set of potential chat-room partners for their subjects: smiley, fictitious teenagers, complete with sham MySpace pages. The setup was that the other kids would eventually tell the subjects in the scanner whether they did or did not want to chat with them. The scans were taken, then, while the subjects were lying still, awaiting the verdict. In a handful of pilot experiments, this has proved to be an emotionally significant challenge for teenagers with social anxiety. The anxious youngsters, while waiting to hear from one of the pretend teenagers they wanted to avoid, showed more reactivity in the amygdala and prefrontal cortex. Pine has conducted this same experiment on 40 of Fox’s longitudinal-study subjects and is currently analyzing the results.

Still, tracking the anxious mind, even with a more realistic experimental setup, means having the subject lie in an M.R.I. scanner, which is inherently not only artificial but also stressful. So Plomin’s point is interesting. Brain scans and other lab findings might reflect something deep and persistent going on in the anxious mind. But if you have learned to control your behavior, to structure your life so you can limit triggers and cope with your emotional skittishness, how much does it really matter?

THE BEHAVIORAL STRAND of the brain-behavior-experience triad is the one that seems most amenable to intervention, and scientists are now investigating how it is that two-thirds of those with a high-reactive temperament manage to avoid trouble. Many environmental factors no doubt come into play — some of them malleable, some less so. In Kagan’s first study, for instance, he found that birth order seemed relevant. Behaviorally inhibited children were much more likely to have older siblings: two-thirds of them did, compared with just one-third of the uninhibited children. Could having older siblings, he and his co-authors wondered, mean being teased and pushed, which becomes a source of chronic stress, which in turn amplifies a biological predisposition to inhibition? Kagan never replicated this finding, as intriguing as it was — which shows how difficult it can be to tease out which environmental factors are relevant, and which turn out to be incidental. Fox, meanwhile, noted that the high-reactive babies who went to day care when they were young were significantly less fearful at age 4 than were the high-reactives who stayed home with their mothers.

Attempts to see what kind of parenting works best with an anxiety-prone temperament leave almost as many questions asked as answered. Which is better for a fearful, high-strung child — a parent who coddles the child and says everything will be all right, or a parent who sets firm, strict limits and has no tolerance for skittishness? You could picture it as going either way, really. On the one hand, it might be good to shield children from the things that worry them. On the other hand, it might be better to urge them, maybe even force them, to confront the things they dread.

Scientists from both Kagan’s and Fox’s labs have looked at this question in a systematic way, and they have come up with two somewhat different findings. Both studies involved a series of home visits and hours of videotapes of mother-baby interactions. But one study, by Kagan’s graduate student Doreen Arcus in the early 1990s, found that what seemed to be best for high-reactive babies were mothers who set firm limits and did not rush too quickly to comfort them when they cried. And the other, by Fox’s postdoctoral fellow Amie Ashley Hane a decade later, found something slightly different: that the best fit for high-strung babies were sensitive mothers, who met their fearful children on their own terms and interacted with them in a way that was accepting and supportive without being intrusive. Sometimes, of course, there’s a fine line between firm and hardhearted, and a fine line between supportive and intrusive. This makes it especially tough to turn research findings like Arcus’s and Hane’s into clear guidance on how best to care for a fretful child.

The best outcome, however it happens, is to rear a child who learns to wrestle his demons on his own. Some children figure out themselves what works best. “Inner struggles pulled at me for years until I was able to just let go and calm myself,” wrote one of Kagan’s high-reactive study subjects in an essay, revealing a wisdom far beyond his 13 years. “For example, when I first heard about the anthrax in Washington, I began to have an upset stomach. I realized it was simply because of my anxiety that I was feeling sick. As soon as I realized that, the stomachache went away. Because I now understand my predisposition toward anxiety, I can talk myself out of simple fears.” There are many adults, anxious or not, who can’t control their own interior monologues as well as this boy can.

For the children who need help grappling with their fears, some psychologists try to intervene early, with programs that give worried children tools for quieting the scary thoughts in their heads. Kids are often taught the same skills that anxious adults are, a variation on cognitive behavior therapy, designed to stop the endless recursive loop of rumination, replacing it with a smart, rational interior voice. In a way, it’s teaching anxious people to do what non-anxious people do naturally.