rely on the difference between MZ and DZ twin correlations, making the model unidentified. Given the absence of sibling or adoption data, separate models including shared environmental and non-additive effects were compared using the Akaike Information Criterion (AIC); however, given the lack of evidence for non-additive genetic effects in the literature (Agrawal and Lynskey, 2008) and in our model comparisons, we only report on the results from our ACE models. Further information regarding ADE models are available upon request from the corresponding author.
