The role of general intelligence as an intermediate phenotype for neuropsychiatric disorders

Introduction. Neurocognitive impairment is common to several neuropsychiatric disorders. The growing use of cognitive impairment as an intermediate phenotype, or endophenotype, in psychiatry raises the issue of whether global measures of cognition, such as IQ, or assays of more specific cognitive domains, such as working memory, will best serve to enhance power in detecting susceptibility loci in molecular genetic studies. Methods. This paper will review the research on general intelligence in schizophrenia and bipolar disorder and evaluate its strengths and weaknesses as a candidate intermediate phenotype. Results. Although global measures of cognition may not be optimal as intermediate phenotypes in bipolar disorder, certain clinical traits that overlap between schizophrenia and bipolar disorder, such as psychosis, may be predictive of poor performance on global measures, regardless of DSM-IV categorisation. Conclusions. Global measures of cognition represent good intermediate phenotypes in schizophrenia. Current research does not support the use of global measures of cognition as intermediate phenotypes for bipolar disorder. Assays of specific neurocognitive domains may have greater potential to detect genetic markers for bipolar disorder.