BackgroundIndividual differences in symptoms of behaviour problems in childhood and adolescence are not primarily due to nature or nurture – another substantial source of variance is non‐shared environment (NSE). However, few specific environmental factors have been found to account for these NSE estimates. This creates a ‘missing NSE' gap analogous to the ‘missing heritability’ gap, which refers to the shortfall in identifying DNA differences responsible for heritability. We assessed the extent to which variance in behaviour problem symptoms during the first two decades of life can be accounted for by measured NSE effects after controlling for genetics and shared environment.MethodsThe sample included 4,039 pairs of twins in the Twins Early Development Study whose environments and symptoms of behaviour problems were assessed in preschool, childhood, adolescence and early adulthood via parent, teacher and self‐reports. Twin‐specific environments were assessed via parent‐reports, including early life adversity, parental feelings, parental discipline and classroom environment. Multivariate longitudinal twin model‐fitting was employed to estimate the variance in behaviour problem symptoms at each age that could be predicted by environmental measures at the previous age.ResultsOn average across childhood, adolescence and adulthood, parent‐rated NSE composite measures accounted for 3.4% of the reliable NSE variance (1.0% of the total variance) in parent‐rated, symptoms of behaviour problems, 0.5% (0.1%) in teacher‐rated symptoms and 0.9% (0.5%) in self‐rated symptoms after controlling for genetics, shared environment and error of measurement. Cumulatively across development, our parent‐rated NSE measures in preschool, childhood and adolescence predicted 4.7% of the NSE variance (2.0% of the total variance) in parent‐rated and 0.3% (0.2%) in self‐rated behaviour problem symptoms in adulthood.ConclusionsThe missing NSE gap between variance explained by measured environments and total NSE variance is large. Home and classroom environments are more likely to influence behaviour problem symptoms via genetics than via NSE.