Chasing Environmental Influences on School Grades in Childhood and Adolescence

School performance in childhood and adolescence is an important indicator for social inequality and various life outcomes in adulthood. Previous research confirmed genetic as well as environmental influences on individual differences in school grades, yet little is known on what lies behind the environmental influences. The aim of this study is to identify external covariates that account for variance in school grades and to disentangle genetic and (non–)shared environmental components in the association between these often assumed “environmental” variables and school grades. The sample consists of 2101 pairs of monozygotic and dizygotic same-sex twins (aged 11 and 17) from the German TwinLife study. Multiple regression analysis showed that our measured external variables (e.g., parental behavior, home environment, peer characteristics) explain about 7–9% of variance in the grade point average (GPA) in both age groups. In order to determine genetic and environmental sources of this variance components, we applied a bivariate Cholesky decomposition. Results indicate that after correcting for parental socio-economic status the relation between external covariates and the GPA is entirely due to shared environmental effects at age 11, while the association between the same set of covariates and GPA at age 17 is due to common genetic sources. This pattern largely remains when considering the covariates individually: Effects are strongest for home environment and negative parental involvement in both age groups and additionally for delinquent peer affiliations at age 17. We discuss possible underlying effects of gene × environment interactions and provide implications for further research.