This article examines evidence cited in favor of the operation of genetic factors in attention-
deficit hyperactivity disorder (ADHD). Like other psychiatric conditions, a belief in the genetic
basis of ADHD is derived from the results of family, twin, and adoption studies. Because
family studies are widely believed to be confounded by environmental factors, primary
emphasis is placed on twin and adoption studies. ADHD twin studies depend on the validity
of the equal environment assumption (EEA), which holds that the environments of identical�…