"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.

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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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