for by each of these factors is performed by comparing within-pair monozygotic twin correlations to within-pair dizygotic twin correlations. Additive genetic effects are indicated when rMZ>rDZ, shared environmental effects when rDZ>0.5rMZ, and non-shared environmental effects when rMZ<1.0. The primary model calculated variance components separately by sex at each age. Additional models were calculated to evaluate goodness of fit in which estimates of the variance components were constrained to be equal across age, sex, or both. Estimates were obtained from observed twin data using a maximum likelihood estimator in the software program Mx. Model fit was evaluated using Akaike’s Information Criterion (AIC), with a lower AIC indicating a better fitting model. Missing data were handled by reading raw data into Mx, and fitting to the observed and unobserved data vectors using full information maximum likelihood estimation (FIML).
