Seeking to clarify the parent-offspring transmission of Major Depression (MD) and type I Bipolar Disorder (BD), we examined offspring MD and BD risk in five informative parental pairs: Unaffected x MD, Unaffected x BD, MDxMD, MDxBD and BDxBD.

We identified 289,637 individuals born in Sweden 1970-1990, followed through 2018, from parents with MD and/or BD identified from Swedish medical registers. We quantified the MD→MD, BD→BD, MD→BD and BD→MD parent-offspring transmission and explored effects of parental illness on MD→BD conversions.

The risk for MD was modestly and similarly increased in offspring of Unaffected x MD (HR=1.64) and Unaffected x BD parents (HR=1.53), higher in MDxMD and MDxBD pairings (HRs=2.39 and 2.47) and slightly lower in BDxBD matings (HR=2.29). By contrast, risk for BD was much higher in Unaffected x BD versus Unaffected x MD matings (HRs = 5.59 vs. 1.70), further elevated modestly in MDxBD matings (HR=6.26) and very high in BDxBD matings (HR=13.61). The rate of offspring MD→BD conversions was substantially increased by parental BD but not parental MD. Offspring BD was equally predicted by paternal and maternal affective illness while offspring MD was more strongly predicted by maternal than paternal affective illness.

Examining risk for MD and BD in offspring of different parental mating types of MD and BD is an informative strategy for further clarifying the cross-generational transmission of these two partially related and partially distinct mood disorders.