TABLE 2. Statistical Comparison of Models 1 Through 6.

Model	Description	−2LL	k	LR	Δdf	p	AIC
Model 1	Saturated model	15,097.344	28	—	—	—	—
Model 2	Means equal for Twins 1 and 2	15,101.134	24	3.790	4	.435	−4.210
Model 3	Means equal for monozygotic and dizygotic twins	15,099.980	24	2.636	4	.620	−5.364
Model 4	Means equal for induction and follow-up	15,171.044	24	73.700	4	<.001	65.700
Model 5	Variances and covariances equal for monozygotic and dizygotic twins	15,340.484	18	243.141	10	<.001	223.141
Model 6	Bivariate ACE model, means differ only between baseline and follow-up	15,122.444	11	25.100	17	.092	−8.900

Note. The table presents the following values: −2 times the log likelihood (−2LL), the number of free parameters in the model (k), the results of a likelihood ratio test (LR, distributed as a chi-square statistic), the change in degrees of freedom of the saturated model and submodel (Δdf), the significance of the chi-square test (p), and an index of the balance between goodness of model fit and parsimony (Akaike's information criterion, AIC). The bivariate ACE model includes additive genetic factors (A), shared family environment (C) and nonshared environment (E).