This article builds upon a proiminent definition of construct validity that focuses on variation in attributes causing variation in measurement outcomes. This article synthesizes the defintion and uses Rasch measurement modeling to explicate a modified conceptualization of construct validity for assessments of developmental attributes. If attributes are conceived as developmental, hypotheses about how new knowledge builds cumulatively upon the cognitive capacity afforded by prior knowledge can be developed. This "cumulative ordering" of knowledge required to accomplish test items constitutes evidence of a specific form of construct validity. Examples of cumulative ordering appear in the extant literature, but they are rare and confined to the early literature. Furthermore, cumulative ordering has never been explicated, especially its relationship to construct validity. This article describes three of the most complete examples of cumulative ordering in the literature. These examples are used to synthesize a method for assessing cumulative ordering, in which the Rasch model is used to assess the progression of item difficulties which are, in turn, used to review developmental theories and hypotheses, and the tests themselves. We discuss how this conceptualization of construct validity can lead to a more direct relationship between developmental theories and tests which, for practitioners, should result in a clearer understanding of what tests results actually mean. Finally, we discuss how cumulative ordering can be used to facilitate decisions about consequential validity.