Table 4.

Model-fitting statistics for the assumption testing of quantitative and qualitative sex differences

Model	Base model	Comparison model	Estimated parameters	-2LL	AIC	df	Change in -2LL	Change in df	p
1	Saturated model		27	2,466.51	452.51	1007	NA	NA	NA
2	Saturated model	Means equated across twin order	23	2,467.38	445.38	1011	0.864	4	0.930
3	Means equated across twin order	Means equated across zygosity	21	2,468.61	442.61	1013	1.231	2	0.541
4	Means equated across zygosity	Means across same sex	19	2,476.11	446.11	1015	7.501	2	0.024
5	Means across same sex	Means across both sexes	18	2,476.11	444.11	1016	<0.001	1	1
6	Means across both sexes	Variances equated across twin order	14	2,477.62	437.62	1020	1.514	4	0.824
7	Variances equated across twin order	Variances equated across zygosity	12	2,477.64	433.64	1022	0.02	2	0.990
8	Variances equated across zygosity	Variances equated across same sex	10	2,478.30	430.30	1024	0.656	2	0.720
9	Variances equated across same sex	Variances equated across both sexes	9	2,480.41	430.41	1025	2.111	1	0.146
10	Variances equated across both sexes	Covariances equated between MZ male and female	8	2,480.41	428.41	1026	0.004	1	0.951
11	Covariances equated between MZ male and female	Covariances equated between DZ male and female	7	2,480.64	426.64	1027	0.229	1	0.632
12	Covariances equated between DZ male and female	Covariances equated across DZ opposite sex	6	2,480.67	424.67	1028	0.028	1	0.868

Alpha criterion is set at p < 0.01. -2LL = minus two log-likelihood, AIC = Akaike Information Criterion, df = degrees of freedom. A lower AIC represents a better model fit. A significant p-value (p < 0.01) indicates a significantly worse model fit.