Heritability of word recognition in middle-aged men varies as a function of parental education

Although it is of lifelong importance, reading ability is studied primarily in children and adolescents. We examined variation in word recognition in 347 middle-aged male twin pairs. Overall heritability (a²) was 0.45, and shared environmental influences (c²) were 0.28. However, parental education moderated heritability such that a² was 0.21 at the lowest parental education level and 0.69 at the highest level; c² was 0.52 and 0.00, respectively. This constitutes a parental education × environment interaction. The higher heritability was due to a decrease in the magnitude of shared environmental factors, rather than an increase in the magnitude of genetic factors. Other cognitive studies have reported gene × environment interactions, but patterns may differ as a function of age or specific cognitive abilities. Our results suggest that shared environmental factors in families with low parental education have long-lasting effects on word recognition ability, well beyond any critical period for developing reading proficiency.