Panic disorder (PD) and agoraphobia are common anxiety disorders that often co-occur. PD is characterized by recurrent panic attacks that include numerous symptoms (eg, palpitations, sweating, sensations of shortness of breath, fear of losing control); agoraphobia is characterized by anxiety associated with being in certain situations from where escape might be dif® cult (eg, crowds, standing in line, bridges, or tunnels) and subsequent avoidance of those situations. These disorders can be disabling. It is common, but not invariant, for agoraphobia to develop in the course of PD. Twin studies (in females) suggest that the heritability of PD is about 44%[Kendler et al., 1995], and the heritability of agoraphobia is 61%(this estimate derived from a study with correction for assessment reliability [Kendler et al., 1999]).