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The next step, Rauch says, is to scan groups of people who are likely to be thrust into dangerous situations--fire fighters, say, or police officers. Then it may be possible to determine if any changes in their brains are the result of traumatic situations or if the changes predate them. Either is plausible. The stress of surviving a building collapse, for example, could turn a normal amygdala into an overactive one. Or an already overactive amygdala may overwhelm the brain in the wake of a disaster.
Eventually, researchers would like to learn what role our genes, as opposed to our environment, play in the development of anxiety. "It has been known for some time that these disorders run in families," says Kenneth Kendler, a psychiatric geneticist at Virginia Commonwealth University in Richmond, Va. "So the next logical question is the nature-nurture issue." In other words, are anxious people born that way, or do they become anxious as a result of their life experiences?
Kendler and his colleagues approached the question by studying groups of identical twins, who share virtually all their genes, and fraternal twins, who, like any other siblings, share only some of them. What Kendler's group found was that both identical twins were somewhat more likely than both fraternal twins to suffer from generalized anxiety disorder, phobias or panic attacks. (The researchers have not yet studied twins with post-traumatic stress disorder or obsessive-compulsive disorder.)
The correlation isn't 100%, however. "Most of the heritability is in the range of 30% to 40%," Kendler says. That's a fairly moderate genetic impact, he notes, akin to the chances that you will have the same cholesterol count as your parents. "Your genes set your general vulnerability," he concludes. "You can be a low-vulnerable, intermediate-vulnerable or a high-vulnerable person." But your upbringing and your experiences still have a major role to play. Someone with a low genetic vulnerability, for example, could easily develop a fear of flying after surviving a horrific plane crash.
There is plenty to learn about how anxiety and fear shape the brain. One of the biggest mysteries is the relationship between anxiety and depression. Researchers know that adults who suffer from depression were often very anxious as children. (It's also true that many kids outgrow their anxiety disorders to become perfectly well-adjusted adults.) Is that just a coincidence, as many believe, or does anxiety somehow prime the brain to become depressed later in life? Brain scans show that the amygdala is very active in depressed patients, even when they are sleeping. Studies of twins suggest that many of the same genes could be involved. "There's a lot of overlap," says Dr. Dennis Charney, chief of the research program for mood and anxiety disorders at the NIMH. "Anxiety and depression have a similar underlying biology, and the genetics may be such that anxiety surfaces early in life and depression later on." Still, no one can say for sure.