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Over the years, twin and family genetic studies have estimated heritability of major depressive disorder at about 37%. These investigators used Swedish national registries (covering approximately 87% of Sweden's population) to refine and extend previous work.
One analysis focused on monozygotic twins, dizygotic twins, full siblings, and half siblings reared together and apart (N=1,718,863 pairs; birth-years,1950–1990). Another analysis focused on pairs of monozygotic twins, full siblings, half siblings, and cousins, of whom one had major depression (N=875,010 pairs; birth-years, 1970–1990). Of probands with depression (mean age at disease onset, 29; mean depressive episodes, 2.3), approximately 50% had an anxiety disorder, 77% were prescribe…