Nonshared environment: Real but random

Abstract Background In the excitement about genomics, it is easy to lose sight of one of the most important findings from behavioural genetics: At least half of the variance of psychopathology is caused by environmental effects that are not shared by children growing up in the same family, which includes error of measurement. However, a 30‐year search for the systematic causes of nonshared environment in a line‐up of the usual suspects, especially parenting, has not identified the culprits. Method I briefly review this research, but primarily consider the conceptual framework of the search for ‘missing’ nonshared environmental effects. Results The search has focused on exogenous events like parenting, but nonshared environment might not be caused by anything we would call an event. Instead, it might reflect endogenous processes such as noisy biological systems (such as somatic mutations and epigenetics) or, at a psychological level, idiosyncratic subjective perceptions of past and present experiences, which could be called nonshared environmental experience to distinguish it from exogenous events. Although real, nonshared environment might be random in the philosophy of science sense of being unpredictable, even though it can have stable effects that predict subsequent behaviour. Conclusion I wade into the weeds of randomness and suggest that this so‐called ‘gloomy prospect’ might not be so gloomy.