One Twin Suffered, the Other Did Not. Their Adult Mental Health Differed. Twins provide a natural experiment for psychologists studying genetics and environment. Dennis and Douglas were similar in high school, but their lives diverged in adulthood. Dennis experienced trauma and struggled with mental illness, while Douglas did not. A study of 25,252 adult twins in Sweden found that childhood trauma increased the likelihood of psychiatric illness. The findings emphasize the impact of childhood adversity on adult mental health, independent of genetic factors. This evidence underscores the need to address childhood abuse and neglect to prevent long-term harm.