Abstract
IMPORTANCE: The nature of the genetic relationship between major depression and bipolar disorder remains unclear and might be clarified by considering disorders outside of the mood spectrum. OBJECTIVE: To better understand the relationship between genetic liabilities for major depression (MD) and bipolar disorder (BD). DESIGN, SETTING, AND PARTICIPANTS: A cohort study was conducted with data for individuals born in Sweden to Swedish parents from 1960 to 1990, with follow-up through December 31, 2018. The data included family genetic risk scores for MD and BD and International Classification of Diseases codes for a range of disorders as reported in primary care, specialist, and hospital registries. Data analysis was conducted from April 2022 to July 2022. EXPOSURES: High and low genetic liability were defined as being in the upper and lower 2 risk deciles. Risk was compared in individuals at high genetic liability to (1) MD only, (2) BD only, and (3) both MD and BD and those at (4) high genetic liability to BD and low genetic liability to MD and (5) high genetic liability to MD and low genetic liability to BD. MAIN OUTCOMES AND MEASURES: Risk for nonpsychotic MD and BD, psychotic MD and BD, anxiety disorders, obsessive-compulsive disorder, schizoaffective disorder (SAD), schizophrenia, and other nonaffective psychosis. RESULTS: Data were included for 2 736 950 individuals with a mean (SD) age at follow-up of 43.9 (9.1) years. High genetic liability to only BD increased risk for nonpsychotic BD, psychotic BD, and SAD. High genetic liability to only MD augmented risk for nonpsychotic MD, anxiety disorders, and nonpsychotic BD. High genetic liability to both BD and MD had the strongest association with risk for nonpsychotic BD, anxiety disorders, and nonpsychotic MD. High genetic liability to BD and low genetic liability to MD increased risk for psychotic BD, nonpsychotic BD, and SAD with no increased risk for nonpsychotic MD or anxiety disorders. High genetic liability to MD and low genetic liability to BD increased risk for nonpsychotic MD, nonpsychotic BD, and anxiety disorders with no increased risk for psychotic BD. CONCLUSIONS AND RELEVANCE: In this study, hypotheses that BD and MD are either genetically distinct or genetically closely interrelated were not supported. Both BD and MD were associated with a genetic vulnerability to mood disorders, but even that liability was partially selective. However, compared with individuals at high liability to MD, those at elevated genetic liability for BD had a substantially increased risk for psychosis. Compared with individuals at elevated genetic liability to BD, those at high genetic risk for MD had a considerably augmented risk for anxiety disorders. Clarifying genetic relationships between psychiatric syndromes can be substantially aided by the consideration of profiles of risk for a range of disorders.