There are long-term, biological, environmental, and social causes of panic attacks. In 1993, Fava et al. proposed a staging method of understanding the origins of disorders. The first stage in developing a disorder involves predisposing factors, such as genetics, personality, and a lack of well-being. Panic disorder often occurs in early adulthood, although it may appear at any age. It occurs more frequently in women and more often in people with above-average intelligence. Various twin studies where one identical twin has an anxiety disorder have reported a high incidence of the other twin also having an anxiety disorder diagnosis.