The Duality of Schizotypy: Is it Both Dimensional and Categorical?

Schizotypy is the notion that schizophrenia-like features can form, in the absence of illness, a temperamental “type” or personality trait. Both typological and characterological accounts were present at the notion’s conception as, historically, both categorical (“Kraepelinian”) and dimensional [e.g., Kretchmer’s “schizothymic” temperament; (1)] accounts of psychotic illness have vied against one another with the former clearly ascendant in biological psychiatry at least. Paul Meehl’s influential development of the categorical account (2) theorized the “schizotype” as the category as the fundamental phenotypic foundation of “true” schizophrenia. Variants of this model remain central to theorizing in the North American tradition at least. The dimensional view, revitalized by Hans Eysenck, is best represented in contemporary theory by Gordon Claridge’s “quasi-dimensional” model (3). In 1995, Adrian Raine and Todd Lencz (4) set out some of the theoretical and conceptual issues in schizotypal personality research and outlined the “categories versus dimensions” issue as “perhaps the most important” of all (p. 5). They suggested pursuing both approaches so as to see, which is most productive. There is of course a distinction here between theory and methodology. I aim to argue here based on four observations of the empirical literature that aspects of both theoretical accounts may be valid, and that a diversity of methods may have utility in the field. Claridge’s dimensional account postulates underlying dimensionality of risk for illness with superimposed clinical discontinuities – the schizophrenic “spectrum” of illnesses. The critical difference between the two accounts of schizotypy lies in the non-clinical portion of the phenotype. In the categorical account, only a portion of phenotypic schizotypy is at genuine elevated risk, the “true” schizotype, the remainder is pseudophenotypic, superficially mimicking schizotypy but not possessing true genetic risk: Adrian Raine (5) termed the latter “pseudo-schizotypal. ” In the dimensional account, by contrast, there is the possibility of “genuine” schizotypy possessing a healthy or adaptive outcome (6); a theme I reprise in my conclusions. A few years ago (7), I conducted a bibliographic analysis of the schizotypy literature that evidenced the growing popularity of empirical research in the field (schizotyp* OR schizoid* OR psychosis prone*), and of experimental studies in particular. In addition, I divided the empirical literature into psychometric and experimental studies, and into those taking a categorical and dimensional approach (based on their statistical treatment). The major growth in the literature has been in experimental studies of which more have taken a dimensional (e.g., correlational) approach (Figure ​(Figure11). Figure 1 Bibliometrics: dimensional and categorical approaches. Clearly, there are advantages and disadvantages to both statistical approaches and this choice does not necessarily imply a strong theoretical preference. For example, most quantitative genetic studies examine correlations as a matter of course. Conversely, studies based on diagnostic procedures usually retain a categorical approach. Moreover, a minority of studies report both statistical treatments, often with broadly commensurate results. Treating schizotypy variables as continuous variables is perhaps sometimes preferred as statistical power in many analyses is likely to exceed dichotomized treatment. This is especially the case if the latter takes seriously the taxonomic prediction of 10–15% of a general population sample (arguably a “median split” is the worst of all possible worlds). Large samples are required if the truly taxonomic approach is to be taken in a multivariate analysis. While this suits some fields such as quantitative genetics, it is not suited to others such as brain imaging. On the other hand, a common strategy is to preselect “schizotypal” and “non-schizotypal” groups via large-scale screening using a psychometric instrument. This usefully reduces the number needed to test experimentally to achieve statistical power. However, the strategy may or may not imply testing of a categorical model – it is also, of course, a strategy of convenience for testing dimensional differences. As a consequence of all these considerations, I would argue that while genuine differences clearly exist between the models theoretically, these are very rarely tested against one another genuinely at the empirical level. Evidence can be found (and is often rehearsed) for both categorical and dimensional positions – even from the same dataset. In some ways, this apparent duality may parallel the famous “wave–particle” duality of quantum theory that suggests that both accounts can be “true” in different ways, and thus seeks to explain a diverse range of observations. At the crudest level, one can observe that broad measurement of trait tendencies tend to produce continua, and narrow “symptom-focused” measures lead to categories. I would like to suggest some important ways that both may have validity and research utility (and I do not claim that this is an exhaustive list). I am not alone in noticing empirical evidence for both positions (8). From reviews of the epidemiological evidence of clinical disorders, Linscott and van Os suggest that there is true continuum to the non-clinical. Where I differ from their position is their suggestion that evidence in the general population suggests a latent categorical structure with “two types of people. ” This structure is generally argued for as a result of attempts to identify a taxon psychometrically. However, the statistical issues of this argument certainly allow for divergent interpretations: the issue of taxonometrics in schizotypy has been much discussed with little resolution (e.g., see Personality and Individual Differences 44:8; 2008). Where I do agree with their position is in viewing schizotypy per se as too narrow a lens, “psychosis proneness” captures the variety of traits relevant to psychotic disorders as a whole.