Parents of high risk subjects defined by levels of monoamine oxidase activity.

Thirty-seven parents of probands with low platelet monoamine oxidase (MAO) activity levels and 38 parents of high MAO probands were examined for MAO activity, past and present psychopathology, and reported psychopathology in other relatives. Results showed generally positive and significant correlations between parents' and children's MAO levels, significantly greater rates of "high MAO related" disorders in parents and relatives of high MAO probands and of "low MAO related" disorders and symptoms in parents and relatives of low MAO probands. Support for a two-directional monoamine hypothesis of affective disorders is suggested.