Table II.

Twin Correlations and Variances and Univariate Twin Modeling Parameters

Variable	N (pairs)		Twin Correlations		Twin Variances		Cross-Twin, Cross-Trait (Alcohol Problems)
	MZ	DZ	rMZ	rDZ	MZ	DZ	rMZ	rDZ
CATSS-9/12
ADHD
Hyperactive/ Impulsive	3,421	4,253	.70 [.66, .73]	.13 [.09, .16]	1.77 [1.61, 1.94]	2.36 [2.20, 2.52]	.11 [.04, .18]	.05 [−.02, .12]
Inattentive	3,421	4,253	.61 [.58, .65]	.11 [.07, .14]	2.50 [2.30, 2.70]	3.15 [2.97, 3.33]	.04 [−.03, .10]	.05 [−.02, .12]
Disruptive behaviors	3,420	4,251	.64 [.59, .68]	.30 [.26, .34]	0.89 [0.79, 1.00]	1.17 [1.06, 1.28]	.08 [.01, .16]	.08 [.003, .15]
CATSS-18
Alcohol problemsa	726	748	.69 [.62, .76]	.33 [.21, .46]	18.92 [15.88, 21.96]	18.69 [15.98, 21.39]	--	--

	Univariate Twin Modeling Parameter Estimates from Best-Fitting Models
	Model	Additive (a2)	Dominant (d2)	Non-shared Environmental (e2)	Sibling Contrast (s)
CATSS-9/12
ADHD
Hyperactive/ Impulsive	AEs	.82 [.79, .85]	--	.19 [.15, .22]	−.16 [−.19, −.14]
Inattentive	AEs	.76 [.72, .79]	--	.24 [.21, .28]	−.15 [−.18, −.13]
Disruptive behaviors	AEs	.68 [.62, .75]	--	.32 [.25, .38]	−.04 [−.08, −.003]
CATSS-18
Alcohol problems	AE	.69 [.63, .75]	--	.31 [.25, .38]	--

Note. CATSS-9/12 reports are provided by caregivers, and CATSS-18 reports are provided by twins. Values are estimate [95% CI]. All parameter estimates control for pair sex. Twin correlations were estimated from models constraining intercepts and variances to equality across twins and zygosity groups. Variances were estimated from similar models without constraints for equal variances across zygosity groups. Unless otherwise noted, all MZ and DZ variances differed significantly from each other, Wald p < .05. Proportions of additive and non-shared environmental variance were calculated independent of the sibling contrast effects (i.e., a² + e² = 1).

^aTwin variances did not significantly differ from each other, Wald p = .91.