True et al (1993) carried out a study of 4042 twins who fought in Vietnam. The researchers found that monozygotic twins had a significantly higher rate of concordance for the disorder than dizygotic twins. In addition to twin research, psychologists also find that PTSD runs in families. A study by Yehuda et al (2001) found that adult children of Holocaust survivors with PTSD had a higher risk of PTSD following trauma...