Changes in heritability and shared environmentality of educational attainment across twentieth-century Norway.

Sociological modernization theory predicts that rising meritocracy reduces the influence of family background on education while increasing the role of individual abilities. Sibling correlations are often used as omnibus indicators of family influence, but they reflect both shared environments and shared genetics, complicating interpretation over time. This study examines changes in the genetic and environmental components of educational attainment using administrative data on more than 13,000 Norwegian twin pairs born between 1915 and 1991. We apply a Bayesian hierarchical liability-threshold model to estimate heritability and shared environmental influences across birth cohorts, accounting for changes in educational systems and population distributions. For women, the heritability of educational attainment increased steadily across the twentieth century, while shared environmental influences declined, consistent with expanding educational opportunities and gender equality. For men, neither heritability nor shared environmental influences changed significantly over time. These findings clarify how genetic and environmental contributions to education vary across historical contexts, refining interpretations of long-term trends in family influence.