The estimated heritability of MDD in the present sample is 27%, somewhat below the 35% to 40% reported in the literature.15-17 The present heritability estimate of 46% for PTSD falls between the 30% estimate reported for the Vietnam Era Twin Registry sample (the all-male sample examined in most twin studies of PTSD) and the estimate of 71% reported for an all-female sample in earlier work by our group.22 The high degree of genetic overlap between MDD and PTSD observed in the present study is also consistent with previous studies based on the Vietnam Era Twin Registry sample.14,20
