Context: Understanding the relative contributions of genetic and environmental factors to trauma exposure, posttraumatic stress disorder (PTSD), and major depressive disorder (MDD) is critical to developing etiologic models of these conditions and their co-occurrence.
Objectives: To quantify heritable influences on low-risk trauma, high-risk trauma, PTSD, and MDD and to estimate the degree of overlap between genetic and environmental sources of variance in these 4 phenotypes.
Design: Adult twins and their siblings were ascertained from a large population-based sample of female and male twin pairs on the basis of screening items for childhood sexual abuse and physical abuse obtained in a previous assessment of this cohort.
Setting: Structured psychiatric telephone interviews.
Participants: Total sample size of 2591: 996 female and 536 male twins; 625 female and 434 male nontwin siblings.
Main outcome measure: Lifetime low- and high-risk trauma exposure, PTSD, and MDD.
Results: In the best-fitting genetic model, 47% of the variance in low-risk trauma exposure and 60% of the variance in high-risk trauma exposure was attributable to additive genetic factors. Heritable influences accounted for 46% of the variance in PTSD and 27% of the variance in MDD. An extremely high degree of genetic overlap was observed between high-risk trauma exposure and both PTSD (r = 0.89; 95% CI, 0.78-0.99) and MDD (r = 0.89; 95% CI, 0.77-0.98). Complete correlation of genetic factors contributing to PTSD and to MDD (r = 1.00) was observed.
Conclusions: The evidence suggests that almost all the heritable influences on high-risk trauma exposure, PTSD, and MDD, can be traced to the same sources; that is, genetic risk is not disorder specific. Individuals with a positive family history of either PTSD or MDD are at elevated risk for both disorders and should be closely monitored after a traumatic experience for symptoms of PTSD and MDD.