Wium‐Andersen MK, Villumsen MD, Wium‐Andersen IK, et al. - Since familial and genetic factors tend to contribute to the development of depression, researchers investigated the effect of familial factors on the risk of depression by age at first diagnosis. Twenty-three thousand four hundred ninety-eight monozygotic and 39,540 same‐sex dizygotic twins who were followed from 1977 through 2011 were included from the population‐based Danish Twin Registry. A total of 1,545 twins were diagnosed with depression during follow‐up. For twins whose first depression diagnosis was at age 35 or younger, heritability was estimated to be 24.8%, while if it was at age 90 it was 14.7%. Whether caused by genes or shared environment, familial risk of depression appeared to decrease marginally with age at diagnosis; the increased concordance risk for monozygotic over same-sex dizygotic pairs indicated a genetic link to the development of depression.