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. Author manuscript; available in PMC: 2013 Mar 1.
Published in final edited form as: Child Dev. 2012 Jan 30;83(2):743–757. doi: 10.1111/j.1467-8624.2011.01721.x

Table A1.

Monozygotic (MZ) and Dizygotic (DZ) twin correlations for academic achievement in lower, middle, and higher income families.

Group Family Income (1–7 scale) Family Income Range (1962 dollars) NMZ NDZ rMZ rDZ h2 c2 e2
Lower SES 1, 2 Less than $5000/year to $7499/year 188 104 .840 .657 36.6% 47.4% 16.0%
Middle SES 3, 4 $7,500/year to $14,999/year 205 144 .882 .626 51.2% 37.0% 11.8%
Higher SES 5, 6, 7 $15,000/year to $25,000/year and over 82 54 .902 .573 65.8% 24.4% 9.8%
Full Sample 1,2,3,4,5,6,7 Less than $5000/year to $25,000/year and over 475 302 .880 .637 48.6% 39.4% 12.0%

Note: h2 = 2(rMZ − rDZ). c2 = rMZ − 2(rMZ − rDZ). e2 = 1−rMZ. h2, c2, and e2 estimates reported here for the full sample differ slightly from those reported in the body of the paper, because results reported in the body of the paper were produced by structural equation models that controlled for the main effects of SES.