Earlier work has provided experimental evidence for the existence of the primary and secondary process mental organization posited by Freud and has demonstrated that primary process effects are the more active unconsciously (Brakel et al. 2000). Primary and secondary processes were assessed by a categorization test in which qualitatively different principles could be used. In new experiments using the same stimuli, another significant implication of Freud's model was tested: that primary process mental organization has developmental priority. In these experiments, which studied 559 participants ranging in age from 3 to 80, it was found (1) that primary process mentation predominates in preschoolers; (2) that it is not until around age 7 that primary process organization is supplanted by secondary process organization; and (3) that after age 7 the predominance of secondary process organization remains remarkably stable throughout the life span.