In this paper, we describe how the Harthill Leadership Development Profile (LDP), a language-based instrument has evolved from Jane Loevinger’s Washington University Sentence Completion Test (WUSCT), and has been redesigned to assess and offer feedback about adults’ action logics in work or educational settings, in the context of Developmental Action Inquiry (DAI) theory, practice, and method (Torbert, 1972, 1976, 1987, 1991; Torbert & Associates, 2004). Next, we challenge a recent critique of the LDP as a soft measure unsupported by published, quantitative psychometric reliability and validity studies (Stein & Heikkinen,2009) and present both previously unpublished and previously published-but-not aggregated studies illustrating Harthill LDP as a well-calibrated measure of adult ego development. Because the DAI approach to social inquiry and social practice invites us all to interweave first-, second-, and third-person inquiry and everyday action, the validity studies reported tend to concern field-based experiments seeking to generate developmentally transforming change in adults, including the researchers and/or interventionists, as well as in the organizations in which they participate. In our conclusion, we briefly consider what a social science and a social practice based on the developmentally late action-logics will look like, once social science is recognized as embracing, not just 3rd-person empirical positivist research “on” subjects, but also 1st-, 2nd-, and 3rd-person research and action with co-participants in live settings.