The extended univariate G × E model is quite flexible. Twins are not required to be concordant on the value of the moderator (although they can be), and the moderator can be either continuous or categorical, although it should include zero. The conflictive parenting variable was thus floored at zero and collapsed into nine groups (from an observed range of 0 to 24.5), so that each level of conflictive parenting contained roughly 100 twins. Although the interpretation of standardized or proportional ACE estimates may be useful in some cases, it is generally recommended that unstandardized or absolute ACE estimates be presented (Purcell, 2002). We thus standardized our log-transformed CP score to have a mean of 0 and a standard deviation of 1 to facilitate interpretation of the unstandardized values.
