1 Subjects and Methods

To illustrate the application of these models we consider an updated set of data first presented by Hewitt, et al., (1990) and now based on 983 families where both parents rated each of their twin children using Achenbach's Child Behavior Checklist (CBC; Achenbach and Edelbrock, 1983). For the full analysis, published in Hewitt et al. (1992), data from a population-based sample of 500 MZ twin pairs and 483 DZ twin pairs were considered and ratings were included irrespective of the biological or social relationship of the parent to the child. The children were Caucasian and ranged in age from 8 to 16 years. Ratings on 23 core items assessing children's internalizing behavior in both younger and older children and in either boys or girls were totalled to obtain an internalizing scale score for each child. The items contributing to this scale are listed in Appendix .

For illustrative purposes in this chapter we just consider the ``prepubertal" subsample of younger children aged 8-11 years. More detailed analyses, including older children, may be found in Hewitt et al. (1992). The scale scores were log-transformed to approximate normality and adjusted for linear regression on age and sex within age cohorts. The observed variances, covariances, and correlations of the resulting scores are given in Table 11.1 by zygosity and sex group.

Table 11.1: Observed variance-covariance matrices (lower triangle) and twin correlations (above the diagonal) for parental ratings of internalizing behavior problems in five zygosity-sex groups (MZ female, N=96; MZ male, N=102; DZ female, N=102; DZ male, N=97; DZ male-female, N=103). All twins were between 8 and 11 years at assessment.

Zygosity/sex group		Twin 1		Twin 2
		Mothers	Fathers	Mothers	Fathers
MZ female	MT1	.694	.47	.84	.46
	FT1	.312	.638	.37	.72
	MT2	.569	.238	.666	.45
	FT2	.308	.461	.293	.647
MZ male	MT1	.675	.40	.74	.43
	FT1	.265	.652	.35	.77
	MT2	.513	.237	.714	.51
	FT2	.292	.513	.354	.676
DZ female	MT1	.565	.41	.55	.29
	FT1	.241	.604	.25	.57
	MT2	.291	.137	.488	.52
	FT2	.171	.347	.285	.604
DZ male	MT1	.621	.47	.70	.34
	FT1	.315	.719	.35	.73
	MT2	.434	.236	.623	.37
	FT2	.233	.531	.251	.743
DZ male-female	MT1	.538	.26	.49	.18
	FT1	.162	.730	.17	.56
	MT2	.243	.102	.465	.37
	FT2	.103	.372	.191	.574