BackgroundNeuropsychological abnormalities in schizophrenia are well replicated and are present in unaffected relatives. Cognitive findings in bipolar disorder are less clearly established.AimsTo examine the possibility that these abnormalities may provide a means by which the disorders might be separated and to clarify the associations of phenotypic expression and genetic liability.MethodA neuropsychological test battery was administered to 50 control participants, 74 patients and 76 unaffected relatives recruited for the study. Patients included those with schizophrenia from families affected by schizophrenia alone, those with bipolar disorder from families affected by bipolar disorder alone and those with bipolar disorder from families affected by both disorders. Unaffected relatives were also recruited.ResultsCurrent, verbal and premorbid IQ were impaired in people with schizophrenia and in their close relatives. Memory was impaired in all patient and relative groups. Psychomotor performance and performance IQ were impaired in patients, regardless of diagnosis.ConclusionsThis study finds evidence that intellectual abnormalities are related to a genetic liability to schizophrenia. Abnormalities of memory appear to be related to an increased liability to psychosis in general. No impairment was specific to bipolar disorder.