Genetic Boundaries of the Schizophrenia Spectrum: Evidence From the Finnish Adoptive Family Study of Schizophrenia

OBJECTIVE: Identification of the genetically related disorders in the putative schizophrenia spectrum is an unresolved problem. Data from the Finnish Adoptive Family Study of Schizophrenia, which was designed to disentangle genetic and environmental factors influencing risk for schizophrenia, were used to examine clinical phenotypes of schizophrenia spectrum disorders in adopted-away offspring of mothers with schizophrenia spectrum disorders. METHOD: Subjects were 190 adoptees at broadly defined genetic high risk who had biological mothers with schizophrenia spectrum disorders, including a subgroup of 137 adoptees at narrowly defined high risk whose mothers had DSM-III-R schizophrenia. These high-risk groups, followed to a median age of 44 years, were compared diagnostically with 192 low-risk adoptees whose biological mothers had either a non-schizophrenia-spectrum diagnosis or no lifetime psychiatric diagnosis. RESULTS: In adoptees whose mothers had schizophrenia, the mean lifetime, age-corrected morbid risk for narrowly defined schizophrenia was 5.34% (SE=1.97%), compared to 1.74% (SE=1.00%) for low-risk adoptees, a marginally nonsignificant difference. In adoptees whose mothers had schizophrenia spectrum disorders, the mean age-corrected morbid risk for a schizophrenia spectrum disorder was 22.46% (SE=3.56%), compared with 4.36% (SE=1.51%) for low-risk adoptees, a significant difference. Within the comprehensive array of schizophrenia spectrum disorders, schizotypal personality disorder was found significantly more often in high-risk than in low-risk adoptees. The frequency of the group of nonschizophrenic nonaffective psychoses collectively differentiated high-risk and low-risk adoptees, but the frequencies of the separate disorders within this category did not. The two groups were not differentiated by the prevalence of paranoid personality disorder and of affective disorders with psychotic features. CONCLUSIONS: In adopted-away offspring of mothers with schizophrenia spectrum disorders, the genetic liability for schizophrenia-related illness (with the rearing contributions of the biological mothers disentangled) is broadly dispersed. Genetically oriented studies of schizophrenia-related disorders and studies of genotype-environment interaction should consider not only narrowly defined, typical schizophrenia but also schizotypal and schizoid personality disorders and nonschizophrenic nonaffective psychoses.