ABSTRACT
This is a clinical follow-up study of 508 former adolescent psychiatric inpatients admitted to hospital between 2001 and 2006 in Northern Finland. The participants were interviewed using the K-SADS-PL and the EuropASI instruments. Until 2016, the national Finnish Care Register for Health Care provided data on psychiatric diagnoses. Using logistic regression analysis, we examined how birth order and childhood family size affected the incidence of schizophrenia spectrum disorder (SSD) and other psychotic disorders by young adulthood. Participants were separated into three diagnostic subgroups 1) participants with SSD (n = 76), 2) participants with a psychotic disorder other than SSD (n = 107) and 3) participants with only one nonpsychotic depressive episode (n = 118). The main conclusions were that large family size in adolescence (six or more children), male sex and father having psychiatric problems or being unemployed, were significantly associated with a higher likelihood of SSD among study participants. Large family size also associated to a higher likelihood of developing other psychotic disorders. Being the first born or an only child reduced the likelihood of psychotic disorders other than SSD. When diagnosing and treating children and teenagers with psychotic symptoms, information regarding their family size and sibling position may be relevant.