Fig. 4.

Estimates of ICCs for SS and OS pairs of twins and nontwin siblings based on test scores in mathematics and reading assessed in grades 3–5 or 6–8 and split by years of maternal education. Siblings are defined as two individuals having the same mother for whom the distance in months between births is the smallest among births to this mother between 1994 and 2002. The ICCs are based on 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/sibling pair and individual levels. ICC is computed as the ratio of between-pair variation to the sum of within- and between-pair variations. n = 299,426 children (24,640 twins and 274,786 singletons) and 1,796,532 children-year observations.