Symptoms first occur before three years of age and persist throughout the life span, although improvements can certainly occur in different areas.
AD is a common disorder, occurring in about one in every thousand people.
Currently, the biological mechanisms that cause AD are unknown in most cases. However, studies indicate that genetic factors may be involved in many of these AD cases. In some families, more than one individual is diagnosed with AD suggesting a genetic component. Other studies show that AD occurs more often in identical twins than non-identical twins than would be expected if genetic factors were not involved. These genes may act independently or together and may also act in concert with other factors.