Behavioral genetic research has demonstrated that shared genetics, not shared environment, makes adults who grew up in the same family similar in personality and psychopathology. The same research affirms the importance of the environment but shows that salient environmental influences in adulthood are not shared by family members
they are unique to the individual. Cognitive traits such as cognitive abilities and educational achievement are thought to be an exception, with half of the environmental variance attributed to shared environmental influences. However, most of this cognitive research has involved children. Here, we show that shared environmental influence on cognitive abilities and educational achievement declines from accounting for 20%-30% of the variance in childhood to 10%-20% in adolescence and to near 0% by early adulthood. Educational attainment (years of schooling) shows lasting shared environmental influence (30%) carried over from decisions made in adolescence to go to university, which shows the greatest shared environmental influence (47%). We conclude that specific cognitive abilities as well as general cognitive ability show moderate shared environmental influence in childhood when children live at home, but this influence disappears as young people make their own way in the world. We propose that random endogenous processes are responsible for nonshared environmental influences on adult cognitive abilities. We discuss the far-reaching implications for understanding the environmental causes of individual differences in cognitive abilities in adulthood. (PsycInfo Database Record (c) 2025 APA, all rights reserved).