Heritability of Functional Literacy: Evidence from a Classical Twin Design

Functional literacy-the ability to apply reading, mathematical, and scientific knowledge in authentic contexts as operationalized by the PISA framework-is a key predictor of educational attainment, labour-market outcomes, and economic growth. Despite extensive behavioral-genetic research on cognitive ability, the heritability of competency-based literacy measures remains largely unexamined, particularly outside Western populations. The present study addresses this gap by providing the first twin-based heritability estimates for PISA-type functional literacy from Russia. Participants were 114 twin pairs (50 monozygotic [MZ], 64 dizygotic [DZ]; age 14-15) identified within a nationally representative sample of 33,050 eighth-grade students assessed on the Russian Electronic School platform. A bifactor item-response theory model derived scores for a general cognitive ability factor and three domain-specific factors (mathematical, reading, and scientific literacy). Classical twin structural-equation modelling decomposed phenotypic variance into additive genetic (A), shared environmental (C), and non-shared environmental (E) components. The AE model provided the best fit across all phenotypes. Heritability estimates were substantial: functional literacy = 0.76 [95% CI: 0.67-0.85], mathematical literacy = 0.72 [0.59-0.84], reading literacy = 0.69 [0.56-0.82], and scientific literacy = 0.71 [0.59-0.82]. Shared environmental contributions were negligible and non-significant in all models. The twin sample was broadly representative of the source population, with only small mean differences in functional and mathematical literacy. These findings extend the behavioral-genetic architecture of cognitive ability to competency-based assessments and demonstrate its robustness in a non-Western educational context. Limitations include modest sample size, photo-based zygosity assignment, and absence of molecular genetic data.