Genetic epidemiology can address a number of questions of practical importance for a search for genes for phobic disorders. With the genetics of the phobic disorders established, studies are beginning to ask more specifically what constitutes the additive genetic component. Genome searches have been reported for agoraphobia, simple phobia, social phobia, and for a broad phenotype that includes these three phobias in addition to other anxiety disorders. Genetic epidemiology indicates that finding genes for the phobic disorders will require samples with considerable statistical power since the heritability's of these disorders are modest. Twin studies indicate that the familial transmission of generalized anxiety disorder (GAD) is due to genes. With regard to GAD, additive genes shared with panic disorder and agoraphobia accounted for 20% of the total variance of GAD; indeed, this shared variance component made up 87% of its additive genetic variance.