"A previous study on the genetics of placebo in social anxiety disorder showed that it is serotonin that is associated to placebo responsiveness and not dopamine, " he said.

BBC: Placebo effect may be 'down to genes'

Anxiety disorders include obsessive-compulsive disorder (OCD), social phobia, and generalized anxiety disorder, and are usually treated with prescription drugs, talk therapy, or both.

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According to the National Institute of Mental Health, 18.1% of American adults suffer from a clinical anxiety disorder, with social anxieties making up the majority.

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Some 34% of the survivors who were trapped inside the Murrah building in Oklahoma City when it was blown up came down with post-traumatic stress disorder, a disease in which patients can suffer from flashbacks, heightened anxiety and social withdrawal, according to surveys done by North.

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