Human psychology is about 50% heritable. Not only is one’s predisposition to depression or alcoholism inherited, but also more surprising traits such as time spent watching TV. One of the oddest of these is the number of stressful life events one will encounter. Partly this is because people differ in their assessment of what qualifies as a stressful event. Identical twins were twice as similar as fraternal twins in their scores on their measure of life events. This same pattern exists within twins reared apart.

However, objective differences also take place. Of 1500 twins, the concordance for divorce was higher in identical than in fraternal (55% versus 16%). Personality accounts for a third of the genetic influence on divorce. Counterintuitively, those who are more joyful, present, emotional, and impulsive are more likely to divorce. The risk is genetically inherited, not environmentally, from parents. About 40% across various adoption studies.