that this finding was affected by methodologic limitations of meta-analytic biometrical genetics, including a lack of power to detect sibling interaction, the confounding of C and D in genetic models using only MZ and DZ twins reared together, and the correction used for contrast effects (a form of rater bias, see below). Any one of these factors could lead to an overestimate of heritability. A more recent analysis, taking the unweighted average approach of Faraone et al.,2 which took further account of these limitations, concluded that 60% of the variance was due to genetic factors with the rest equally split between C and E.3
