Psychological Review1985, Vol. 92. No-3. 317-349Copyright 1985 by the American Psychological Association, Inc.0033.295X/85/S00.75"To Carve Nature at Its Joints": On the Existenceof Discrete Classes in PersonalitySteve Gangestad and Mark SnyderUniversity of MinnesotaIn 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, aprevailing and rarely questioned assumption that the units of personality arecontinuous dimensions and an accompanying prejudice against class variables.We examine this prejudice, the arguments that generated it, and those that upholdit. We conclude that these arguments are applicable to class variables as theyoften have been explicated, in phenetic terms; by contrast, genetically explicatedclass variables are not vulnerable to these arguments. We propose criteria forconjecturing and present methods for corroborating the existence of class variablesin personality. Specifically, we test a class model of a construct whose conceptualstatus makes it reasonable to evaluate whether or not the differences betweenindividuals represented by this construct constitute discrete classes. Finally, weexamine the implications for conceptualizing and investigating the nature andorigins of personality.As a psychological concept, personality re-fers to regularities and consistencies in thebehavior of individuals and to structures andprocesses that underlie these regularities andconsistencies. Such phenomena, to the extentthat they exist, ought to distinguish individualsfrom other individuals and to render theiractions predictable. Typically, in personalitytheories, these distinguishing features havebeen treated as comparative individual differ-ences on the assumption that one can mean-The research and preparation of this manuscript weresupported in part by National Science Foundation GrantBNS 82-07632 to Mark Snyder, in part by a NationalScience Foundation Graduate Fellowship to Steve Gan-gestad, and in part by a grant from the University ofMinnesota Computer Center.We are grateful to Auke Tellegen for recommendingthe Monte Carlo simulation and valuable additionalexternal validation analyses; in addition, he and Allan R.Harkness provided helpful advice on conducting thesimulation. Also, we thank Jeffry A. Simpson for assistingin the empirical phase of the real variable control.Finally, we appreciate the constructive comments of PaulE. Meehl, Auke Tellegen, Jack Block, William G. Gra-ziano, Allan R. Harkness. Jeffry A. Simpson, DaveSmith, and Jason Young.Requests for reprints should be sent to Steve Gangestador Mark Snyder, Department of Psychology, Universityof Minnesota, 75 East River Road, Minneapolis, Min-nesota 55455.ingfully compare any two individuals in termsof the same variables.In principle, these personality variablesmay be of at least two varieties. On the onehand, there are dimensions or characteristicsthought to be possessed in some quantitativedegree by all individuals. As a result, thedistributions of such characteristics are con-tinuous ones. Dimensions hardly need furtherintroduction; they are well-known in the tra-ditional vocabulary of personologists as traits.Indeed, most existing comprehensive person-ality inventories purport to measure the basicdimensions of personality. For instance, dom-inance is a putative trait, assumed to bedistributed continuously and measured byone of the scales of the Personality ResearchForm (Jackson, 1974).On the other hand, certain units of person-ality may not be dimensions, but rather, classvariables. Class variables are expressed notcontinuously, but as differences distributedinto discrete categories, and thus are differ-ences that can be introduced by the collo-quialism "There are two (or, for that matter,any finite number of) types of people in theworld." That is, when it comes to personality,class variables seek, as Plato put it, "to carvenature at its joints" by identifying true dis-cretenesses in personality.317318STEVE GANGESTAD AND MARK SNYDERIf comparative individual differences canbe distributed either as continuous dimensionsor as discrete classes, then we may ask whetherany specific difference between individuals isproperly conceptualized as a dimension or aclass variable. Is, for instance, extraversion acontinuous dimension, possessed in some de-gree by all individuals (e.g., Eysenck, 1953),or a class variable, with individuals belongingeither to the discrete class of extraverts or tothat of introverts (e.g., Myers, 1962)? A littlereflection, however, reveals that the appropri-ate roles of continuous dimensions and dis-crete classes in personality rarely ever surfacesas an issue for theoretical or empirical inquiry.The reason, it seems, is that this fundamentalissue is almost universally a prejudged one.With few exceptions, the prejudgment is onecaptured by a statement borrowed from Mil-ton: "Differing but in degree, of kind thesame." Overwhelmingly, the basic units ofpersonality are presumed to be dimensions.In this article, we first examine this pre-judgment, the arguments that generated itand those that uphold it. We conclude thatthere are cogent arguments against class vari-ables as they have commonly been explicated.We next discuss a different explication ofclass variables, one that is not vulnerable tothese same arguments. We then propose cri-teria for conjecturing and methods forcorroborating the existence of class variablesin personality. Specifically, we test a classmodel of a construct whose conceptual evo-lution makes it reasonable to evaluate whetherit identifies discrete classes of individuals.Finally, we examine implications of classmodels for understanding the nature and theorigins of personality.Presumption of Dimensionalism and thePrejudice Against Class VariablesMost personologists immediately recognizethat the presumption of dimensionalism isno straw person. One need not look far tofind explicit statements of the preferred statusof continuous dimensions, for example, theassertion thatThe method of choice in personality scale constructionis one which focuses on a linear relation between itemsand a single underlying latent continuum. Other ap-proaches, embodying more complicated models such as. . . class models have the status of curiosities (Jackson,1971, p. 239).Neither is it difficult to find explicit