Background. Patients may present with cognitive impairment in the euthymic phase of affective disorder, but it is unclear whether the impairment is prevalent before onset of the illness. The aim of the present study was to examine the hypothesis that genetic liability to affective disorder is associated with cognitive impairment.

Method. In a cross-sectional high-risk case–control study, healthy monozygotic (MZ) and dizygotic (DZ) twins with (High-Risk twins) and without (the control group/Low-Risk twins) a co-twin history of affective disorder were identified through nationwide registers. Cognitive performance of 203 High-Risk and Low-Risk twins was compared.

Results. Healthy twins discordant for unipolar disorder showed lower performance on almost all measures of cognitive function: selective and sustained attention, executive function, language processing and working and declarative memory, and also after adjustment for demographic variables, subclinical symptoms and minor psychopathology. Healthy twins discordant for bipolar disorder showed lower performance on tests measuring episodic and working memory, also after adjustment for the above-mentioned covariables. The discrete cognitive impairment found seemed to be related to genetic liability, as the MZ High-Risk twins showed significant impairment on selective and sustained attention, executive function, language processing and working and declarative memory, whereas the DZ High-Risk twins presented with significantly lower scores only on language processing and episodic memory.

Conclusions. The hypothesis that discrete cognitive impairment is present before the onset of the affective disorder and is genetically transmitted was supported. Thus, cognitive function may be a candidate endophenotype for affective disorders.