Bivariate genetic model-fitting: model fit.

Note. −2LL = twice the negative log-likelihood, the difference between −2LL of two models is subjected to chi-square (χ2) distribution. Two fit indices are reported: change in chi-square (Δχ2) and Bayesian Information Criterion (BIC). Δdf = change in degrees of freedom (df). A = additive genetic effects; C = shared environmental effects; E = non-shared environmental effects. E, CE, and AE models are nested within ACE model. The best fitting model is underlined.