Fig. 1.

Maternal years of education and average achievement test score for combined twin and sibling pairs sample. The means of gender-standardized test scores in mathematics and reading over maternal years of education (separated by age group and test type) are shown. The sample includes all twin pairs and closely spaced sibling pairs with available test scores. A closely spaced sibling pair is defined as two siblings having the same mother for whom the distance in months between births is the smallest among births to this mother between 1994 and 2002. n = 299,426 children (24,640 twins and 274,786 singletons) and 1,796,532 children-year observations.