Background. Impairments in verbal learning and memory, executive functions and attention are manifest in some euthymic patients with bipolar disorder (BPD). However, evidence is sparse on their putative role as aetiologically important genetic vulnerability markers for the disorder. This population-based study examined the cognitive functions of affected and unaffected individuals in families with BPD. The aim was to discover whether any cognitive function would indicate genetic liability to the disorder and could thus be regarded as endophenotypes of BPD.

Method. A diagnostic interview and a neuropsychological test battery were administered to 32 familial bipolar I disorder patients, 40 of their unaffected first-degree relatives and 55 controls, all representing population-based samples.

Results. Unaffected first-degree relatives showed impairment in psychomotor performance speed and slight impairment in executive function. Bipolar patients were impaired in verbal learning and memory compared with unaffected relatives and controls. They also differed from controls in tasks of executive functions. There were no difference between the groups in simple attention and working memory tasks.

Conclusions. Impaired psychomotor performance speed and executive function may represent endophenotypes of BPD, reflecting possible underlying vulnerability to the disorder. Verbal memory impairments appear to be more related to the fully developed disorder.