TABLE 1.

Descriptive Statistics and Sibling Intraclass Correlations by Group

	MZ		DZ		Ad		Sibling Intraclass r
Measure	M	SD	M	SD	M	SD	MZ	DZ	Ad
g	101.7	13.6	101.1	12.3	101.2	15.4	.75*	.54*	−.17
Letter Identification	102.7	9.0	102.3	8.7	99.1	14.3	.72*	.60*	.25
Word Identification	103.1	17.5	105.6	18.9	104.9	17.1	.89*	.55*	.22
Word Attack	103.2	12.7	104.0	11.9	102.2	16.3	.82*	.54*	.30*
Phonological Awareness	36.6	13.2	35.8	12.9	36.4	17.3	.91*	.66*	.32*
Reading Comprehension	100.5	14.8	95.7	13.8	–		.80*	.54*	–
Rapid Automatized Naming	84.1	31.7	85.5	36.0	–		.78*	.54*	–

Note. g = general cognitive ability; MZ = monozygotic twins; DZ = dizygotic twins; Ad = genetically unrelated adoptive siblings. Phonological Awareness is expressed in number of correct items (total possible correct for 6 subtests, 10 items each = 60), whereas Rapid Automatized Naming is expressed in number of seconds to complete both Letter and Number Naming subtests. Both composites were residualized for age and gender prior to biometric analyses.

^*p < .05.