Sibling complexity and psychiatric disorders in late adolescence among the Finnish 2000 birth cohort

BACKGROUND: Due to the increasing family instability and multipartner fertility in Western countries, more children have half- and stepsiblings. While numerous studies link family instability and complexity with mental health problems, the focus has typically been on the parents while a small but growing literature suggest that sibling constellations with half- and stepsiblings may also be important for adolescent mental health. METHODS: This study is based on administrative register data of all children born in Finland in 2000 and residing there at ages 0 − 15 (n = 56,372). We identified the biological parents, their cohabiting partners, and any full, half and step siblings that the index child has from birth to age 15. We used Cox proportional-hazards models to compare differences in psychiatric disorders from age 16 to 19 for boys and girls by different sibling constellations. Psychiatric disorders were identified as any disorders treated in specialized outpatient or hospital care (n = 7,999). RESULTS: Almost one in five children in the birth cohort had a cohabiting half sibling and almost 4% had a step sibling. Psychiatric disorders in late adolescence were more prevalent among children in different sibling compositions with half or step siblings. For example, the preliminary results show that adjusting for confounders such as socioeconomic status and living with both biological parents or not, boys and girls with an older half sibling had a 44% (95% confidence interval 34-55) higher risk of psychiatric disorders when compared to children with biological siblings only. The corresponding difference in risk for those with younger half siblings was 24% (11-55). CONCLUSIONS: Psychiatric disorders were more prevalent in late adolescence among children with half and/or step siblings. Future studies on family complexity are likely to benefit from including information on sibling characteristics in addition to those of the parents. KEY MESSAGES: • Adolescents growing up with half or stepsiblings had a higher prevalence of psychiatric disorders in late adolescence. • More studies on sibling composition and adolescent psychiatric disorders are needed to inform the type of supportive measures that may benefit children living with half and stepsiblings.