Background: Intellectual and developmental disability (IDD) researchers have been relatively slow to adopt the search for moderators and mediators, although these variables are key in understanding how and why relationships exist between variables. Although the traditional method of causal steps is useful for describing and understanding moderators and mediators, it is not sufficient for statistical analysis. Methods: The theoretical and statistical processes of evaluating moderators and mediators are explained in terms familiar to IDD psychologists, using examples from IDD literature. Moderator and mediator analyses in five leading IDD journals are assessed for patterns of usage. Results: Although the number of publications in the past decade exceeds previous years, the field is still behind others in both the quantity and quality of the use of moderators and mediators. Conclusion: The field as a whole will advance if the recent theoretical and technical advances outlined in this paper are employed. (Contains 3 figures and 1 table.)