In Reply.—We thank Dr Carlson for drawing our attention to her review article. It was published in Psychiatric Clinics of North America, which is not indexed in the most commonly used bibliographic data bases, and consequently did not appear in our computerized literature searches.
Dr Carlson rightly points out that there is overwhelming evidence to refute the hypothesis that the mentally retarded lack the requisite ego maturation for depression and mania to develop. An intriguing question to us is whether such disease entities as Down's syndrome and phenylketonuria (which have clearly defined biochemical abnormalities) alter the incidence of affective disorders in symptomatic populations.1 Possible interactions between the pathophysiology of the mood disorders and the metabolic disorders associated with mental retardation may provide valuable information regarding the biology of affective illness.