Table 1.

Modified ACE variance components model

Test	Grades	A (additive genetic)	M (SES measure)	C (common environment)	E (nonshared environment)	A′ (genetic × SES)	95% CI for A′		n
							Lower	Upper
Math	3–5	0.590	0.143	0.052	0.209	−0.101	−0.120	−0.082	34,432
Reading	3–5	0.619	0.149	0.003	0.224	−0.030	−0.073	0.013
Math	6–8	0.628	0.169	0.000	0.181	−0.050	−0.110	0.010	21,653
Reading	6–8	0.594	0.166	0.017	0.197	−0.077	−0.130	−0.023

This table is based on variance components models used in Purcell (30) and Tucker-Drob and Bates (16). The first two models (grades 3–5) use a relatedness value of 0.76, whereas the latter two models (grades 6–8) use a relatedness value of 0.77. Relatedness is defined as (n − M)/n + 0.5(M/n), where n is the number of SS twins and M is the number of OS twins. The first two columns list test and test grades. Subsequent columns present variances due to additive genetic effects (A); SES measure, which in our case is years of maternal education at birth (M); shared environment (C); and nonshared environment (E). A′ is a moderator term of interest for additive genetic effects displayed together with lower and upper bounds of 95% CIs in the following two columns. The Scarr-Rowe hypothesis requires A′ to be positive and statistically significant. The last column presents sample sizes used in estimation.