Social Risk or Genetic Liability for Psychosis? A Study of Children Born in Sweden and Reared by Adoptive Parents

Objective: Recent studies suggest a role for social factors during childhood in the later development of schizophrenia. Since social conditions in childhood are closely related to parental psychiatric illness, there is a need to disentangle how genes and social environmental factors interact. Method: A total of 13,163 children born in Sweden between 1955 and 1984 and reared in Swedish adoptive families were linked to the National Patient Register until 2006 regarding admissions for nonaffective psychoses, including schizophrenia. Hazard ratios for nonaffective psychoses were estimated in relation to three indicators of socioeconomic position in childhood (household data of the rearing family obtained via linkage to the National Censuses of 1960-1985) and in relation to indicator of genetic liability (biological parental inpatient care for psychosis). In addition, the total Swedish-born population was investigated. Results: Increased risks for nonaffective psychosis were found among adoptees without biological parental history of psychosis) reared in families with disadvantaged socioeconomic position, which consisted of adoptive parental unemployment (hazard ratio = 2.0), single-parent household (hazard ratio = 1.2), and living in apartments (hazard ratio = 1.3). The risk was also increased among persons with genetic liability for psychosis alone (hazard ratio = 4.7). Among those exposed to both genetic liability and a disadvantaged socioeconomic situation in childhood, the risk was considerably higher (hazard ratio = 15.0, 10.3, and 5.7 for parental unemployment, single-parent household, and apartment living, respectively). Analyses in the larger population supported these results. Conclusions: The results indicate that children reared in families with a disadvantaged socioeconomic position have an increased risk for psychosis. There was also some support for an interaction effect, suggesting that social disadvantage increases this risk more in children with genetic liability for psychosis.