As academic achievement can have a major impact on the development of social inequalities we set out to explore how performance differences arise. Using data of the German twin study TwinLife, genetic and environmental effects on school grades in mathematics, German and the grade point average in two age cohorts (11 and 17 years old) were identified. Structural equation modelling on the data of 432 monozygotic and 529 dizygotic twin pairs as well as 317 siblings of the twins showed substantial genetic effects (up to 62%) in both cohorts on all three variables. Next to genetic influences, the twin-specific environment as well as non-shared environmental influences were found to explain the interindividual differences in mathematics and German as well as the grade point average. A cohort effect showing itself in higher heritability in the older cohort was found for mathematics and the grade point average but not for German. Moreover, we compared twins who were assigned to the same classroom to those twins who were assigned to different classrooms and found lower effects of the twin-specific shared environment in the latter group. Our study thereby contributes to the understanding of the etiology of interindividual differences in academic achievement in the numeracy and literacy domain in two age cohorts.

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Early exposure to home chaos relates to poor school performance. This is often interpreted as a causal influence in educational research, however underlying common genetic and environmental factors might affect both constructs, thus confound the effect. Genetics explain about 60% of variation in school performance, while (non-) shared environment constitutes the remaining variance. Also differences in home chaos – often considered an “environmental” factor – are partially influenced genetically. Thus, we investigate the presumed causal effect of home chaos on school grades, while controlling for genetic and environmental confounders. We analyzed longitudinal data on school grades and home chaos in the TwinLife study (twins aged 11 and 13). Applying a biometric cross-lagged model allowed us to combine variance decomposition with estimating stability, correlational and cross-lagged paths while controlling for genetic and environmental confounders. Results suggest that genetic confounding fully explains the effects of chaos on grades, also implying mechanisms of gene-environment interplay.

Fluid intelligence and conscientiousness are the most important predictors of school grades. In addition to this main effect, researchers have suggested that the two traits might also interact with each other in the prediction of school success. A synergistic and a compensatory form of interaction have been suggested, but past evidence has been mixed so far. Most previous studies on this subject have been cross-sectional and many of them focused on older adolescents or adults in upper secondary school or university. We thus investigated the main and interaction effects of fluid intelligence and conscientiousness on school grades in math and German in a longitudinal sample of 1043 German students from age 11 to 15 years. Results from latent growth curve models with latent interaction terms showed a small compensatory interaction effect for baseline levels of math grades but not for their development. No interaction effect was found for German grades. These findings are discussed against the background that (synergistic) interaction effects between intelligence and conscientiousness might be more relevant in older students from higher secondary school or university context.

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