Some important limitations of the present study need to be acknowledged. First, data from the ongoing longitudinal study were available for only a limited age range of 12–14 years and it remains to be seen how the propensity for DD and genetic influences on this measure will change during mid- and late adolescence. Second, this study used a simple categorical measure of DD which did not allow us to model the DD function and estimate its quantitative characteristics. Finally, the results of model fitting must be interpreted with caution because our relatively modest sample size resulted in limited power to identify possible differences in male and female heritability.
