Fear May Be in the Genes

Twin study offers clues about anxiety disorders

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THURSDAY, July 31, 2003 (HealthDayNews) -- A study of twins suggests genes have a significant influence on how people learn to experience fear.

"Genes seem to account for about half of the individual variation in people's response to this fear conditioning," says Dr. John M. Hettema, an assistant professor of psychiatry at the Virginia Institute for Psychiatric and Behavioral Genetics at Virginia Commonwealth University in Richmond. He is part of a team reporting the findings in the July issue of the Archives of General Psychiatry.

Building on the research, it may be possible to discover "what are the genetic factors that predispose people to anxiety disorders," Hettema adds.

In many cases, fear is good, since it keeps people from getting hurt. But for those with anxiety disorders, irrational fears can be disabling, Hettema says.

"People with anxiety disorders end up having these disorders interfere with their normal life," he says. A person may walk into a mall and have a panic attack even though "there is nothing threatening in the mall," he explains.

In the study, Hettema and his colleagues examined the effects of genes on fear in 173 pairs of twins, about equally divided between fraternal and identical twins.

Twins are often used to study genetics since identical twins share all the same genes. Fraternal twins, however, have about as many genes in common as any other pair of siblings.

When you have a sample of twins, "you can parse out the effect of genes and environment on the trait you're studying," Hettema says.

Collaborating with researchers at Uppsala University in Sweden, the researchers examined how twins responded to "fear conditioning," an experimental technique that teaches people to fear something that is actually harmless.

Twins were shown photos of typically scary things, such as snakes and spiders, but they were also shown images of circles and triangles, which would not be expected to induce fear.

But the researchers gave volunteers a mild electrical shock when they were shown some, but not all, images. In this way, the twins were conditioned to experience fear in response to some images simply because they had been shocked when they saw the images before.

The researchers found identical twins were more likely to have similar responses to the fear conditioning experiment than fraternal twins. Since identical twins share all the same genes, the similarity in their reactions supports the idea that genes play a significant role, Hettema says.

In a previous study of twins in Virginia, Hettema's team found genes seemed to account for about 50 percent of anxiety disorders.

The next step, Hettema says, is to look for the genes that predispose people to anxiety disorders. This can be done, he says, by comparing the genes of people who are highly susceptible to fear conditioning with people who are not.

The identification of anxiety-related genes "allows you to zero in on the biologic processes in the brain that cause these disorders," he notes.

Eventually, this approach could lead to new targets for anti-anxiety drugs, Hettema adds.

He stresses, however, that "this is very basic research." Any new therapies are at least five to 10 years away.

The study is significant, particularly because it demonstrates "that there really is a genetic component to how anxious people may be," says Dr. Mark Barad, an assistant professor of psychiatry and biobehavioral sciences at the Brain Research Institute at the University of California at Los Angeles (UCLA).

"How big that influence is still remains to be determined," Barad adds.

And it is unlikely that a single gene explains individual differences in fear conditioning. "There are probably a number of genes involved, not just one," Barad says.

The UCLA researcher also points out that in the study genes did not account for the majority of the variation in fear conditioning. This suggests, Barad says, that environmental factors, such as upbringing and individual experience, may play an important role, too.

More information

Read more about anxiety from the Anxiety Disorders Association of America or the National Institute of Mental Health, which also has a page devoted to fear.

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