Unique environmental effects increased at age 18–20, the age at which individuals commonly leave the parental home, and remained of similar magnitude at later ages. The variance explained by environmental factors specific to an individual increased from 15% in adolescence to 40–48% during early adulthood. Whereas at younger ages unique environmental influences were largely age-specific, from age 27–29 onwards the largest part of unique environmental influences resulted from those experienced earlier which were then transmitted to a later age. These unique environmental influences can represent any influences that the twins do not have in common, like a specific group of friends, relationships, life events, or working environment.
