Table 1.

Descriptive Statistics and Sibling Intraclass Correlations Among Monozygotic (MZ) and Dizygotic (DZ) Twins

	MZ		DZ		MZ	DZ
Variable	M	SD	M	SD	r	r
Phonological awareness	36.2	13.3	34.8	13.1	.85	.51
RAN	164.2	61.4	166.1	66.2	.55	.37
Letter ID	102.9	9.0	102.2	8.7	.74	.57
Word ID	103.6	18.3	104.7	18.6	.89	.57
Phonological decoding	103.4	12.9	103.3	11.9	.82	.53
Months of school	6.8	6.7	6.7	5.5

Note. Phonological awareness is expressed in number of correct items (total possible correct = six subtests, 10 items each = 60), whereas rapid automatized naming (RAN) is expressed in number of seconds to complete both letter and number subtests of the Comprehensive Test of Phonological Processing. Phonological awareness was residualized for age and gender and RAN for age, gender, and letter identification for all additional analyses. Letter ID and Word ID refer to the Letter Identification and Word Identification subtests of the Woodcock Reading Mastery Tests—Revised.