ABSTRACT Despite widespread interest in individual life histories, few biographies of scientists make use of insights derived from psychology, another discipline that studies people, their thoughts, and their actions. This essay argues that recent theoretical work in psychology and tools developed for clinical psychological practice can help biographical historians of science create and present fuller portraits of their subjects' characters and temperaments and more nuanced analyses of how these traits helped shape their subjects' scientific work. To illustrate this thesis, the (...) essay examines the early career of James McKeen Cattell—an influential late nineteenth‐ and early twentieth‐century experimental psychologist—through a lens offered by psychology and argues that Cattell's actual laboratory practices derived from an “accommodation” to a long‐standing “cognitive deficit.” These practices in turn enabled Cattell to achieve more precise experimental results than could any of his contemporaries; and their students readily adopted them, along with their behavioral implications. The essay concludes that, in some ways, American psychology's early twentieth‐century move toward a behavioral understanding of psychological phenomena can be traced to Cattell's personal cognitive deficit. It closes by reviewing several “remaining general questions” that this thesis suggests. (shrink)