Spontaneously occurring abnormal behaviors in animals have recently received considerable attention, both in veterinary medicine and as a potential model for abnormal behavior in several human mental disorders. Stereotypies are abnormal repetitive, unvarying, and functionless behaviors that are often performed by captive and domesticated animals housed in barren environments. They closely resemble the stereotypies of autistic and mentally retarded patients, stereotypies of unmedicated chronic schizophrenic patients, certain classes of simple tic in Tourette's syndrome, and several drug-induced behaviors. However, evidence for a common mechanism has been lacking. Stereotypies in human mental disorders are indicative of profound brain dysfunction involving the basal ganglia, and are associated with pervasive voluntary-motor impairments and psychological distress. Here we show that stereotypy in captive Orange-Wing Amazon Parrots (Amazona amazonica) is correlated with poor performance on the same psychiatric task (the 'gambling task') as stereotypy in autistic and schizophrenic patients. The task measures recurrent perseveration-the tendency to inappropriately repeat responses. Thus, the more stereotypy a parrot performed, the more likely it was to inappropriately repeat itself from trial-to-trial on the task; and the more rapidly it made repeated, but not switched, responses. These results parallel the executive motor impairments seen in human patients, and therefore suggest that, like in human patients, stereotypy in caged parrots reflects a general disinhibition of the behavioral control mechanisms of the dorsal basal ganglia. If this result holds true in other laboratory species, stereotypic animals are likely to be of questionable utility in behavior, neuroscience, and neuropharmacological experiments. In humans, stereotypies and obsessive-compulsive behaviors are considered to be mutually exclusive categories of behavior, with different neural substrates, and different treatment strategies. These results, therefore, suggest that the pharmacological treatment of stereotypies in veterinary medicine based on the assumption that they are equivalent to human Obsessive-Compulsive Disorder may be inappropriate. As stereotypies in captive animals develop in response to the captive environment, these results also emphasize the role that the environment may play in eliciting or exacerbating stereotypy in human patients. Finally, by parallel to human patients, there is a potential psychological distress in animals showing these behaviors.