"To Carve Nature at Its Joints": On the Existence of Discrete Classes in Personality

In principle, units of personality may be of two varieties: dimensional variables, which involve continuously distributed differences in degree, and class variables, which involve discretely distributed differences in kind. There exists, however, a prevailing and rarely questioned assumption that the units of personality are continuous dimensions and an accompanying prejudice against class variables. We examine this prejudice, the arguments that generated it, and those that uphold it. We conclude that these arguments are applicable to class variables as they often have been explicated, in phenetic terms; by contrast, genetically explicated class variables are not vulnerable to these arguments. We propose criteria for conjecturing and present methods for corroborating the existence of class variables in personality. Specifically, we test a class model of a construct whose conceptual status makes it reasonable to evaluate whether or not the differences between individuals represented by this construct constitute discrete classes. Finally, we examine the implications for conceptualizing and investigating the nature and origins of personality.