Genetic Theory Genetic studies have produced solid evidence that at least some genetic component contributes to the development of anxiety disorders. In 1996, researchers at the National Institute of Mental Health determined that the gene 5-HTTP influences how the brain makes use of serotonin. Statistics indicated that the gene caused a 3% to 4% difference in the degree of anxiety or tension the subjects experienced. Findings from this same study were also used to explore the origins of normal and pathological personality patterns. Family studies have been conducted to determine the prevalence of anxiety in relatives. Two methods are generally used: the family history, which relies on indirect interviews with an informant, and the family study, which is based on direct interviews of family members. These methods have been used to explore theories regarding various classifications of anxiety For example, almost half of all clients with panic disorder have at least one affected relative; about 15% to 20% of individuals with obsessive-compulsive disorder come from families in which another immediate family member has the same problem; and about 40% of people with agoraphobia have a relative with agoraphobia. Some studies have suggested that a relatively simple genetic model may explain the genetic or inheritance pattern of anxiety. The hypothesis states that there are some genes that play a major role, contributing to the manifestation of clinical symptoms of anxiety. For example, recent data provide strong evidence that chromosome 9 may be linked to the development of panic disorder; chromosome 13q may be linked to a potential subtype of panic disorder called “panic syndrome”; and significant linkage was found at chromosome 14 for simple phobia, and possible linkages for social phobias, panic disorder, and agoraphobia. Data regarding potential linkages at chromosome 1 for panic disorder, chromosome 3 for agoraphobia, chromosome 11 for panic disorder, and chromosome 16 for social and simple phobia have also been reported.