Path coefficients corresponding to these genetic and environmental factors were estimated using maximum likelihood, and the goodness of model fit was indicated by −2LL (log likelihood). Submodels were tested by dropping individual paths from the full model. The significance of individual paths was tested by comparing the fit of the submodel to the fit of the saturated model using a χ2 test of the difference between the log likelihoods for the two models with degrees of freedom corresponding to the difference in the degrees of freedom between two models. Model fit was assessed using the Root Mean Square Error of Approximation (RMSEA; equation per Chen et al. 2008; Kenny and McCoach 2003; Rigdon 1996) and Akaike’s Information Criterion (AIC). If dropping a path reduced the goodness of fit (i.e. the RMSEA and AIC increased), the path was retained in the model, otherwise the more parsimonious model was chosen. Heritability was estimated as the percentage of the total variance of the trait attributed to genetic factors; in addition, 95% confidence intervals of the estimates were computed. A detailed description of the model fitting approach and assessment of heritability can be found elsewhere (Neale and Cardon 1992; Rijsdijk and Sham 2002).
