Fig. 3.

Estimates and 95% CIs of between- and within-pair variations for SS and OS twin pairs and achievement test scores in mathematics and reading assessed in grades 3–5 or 6–8 and split by years of maternal education. The variances were obtained using multilevel mixed-effects linear regression estimated with maximum likelihood, where within-individual across-grades errors are assumed to have an autoregressive structure of order one. Random effects are structured at the twin pair and individual levels. n = 24,640 twins and 147,828 children-year observations.