دیدگاه ابعادی اختلال شخصیتی: یک بررسی از شواهد تاکسومتریک

Abstract The dimensional view of personality disorders (PDs) represents these conditions as extreme variants of normal personality continua. This widely held view underpins efforts to characterize PDs in terms of established systems of personality description and to overhaul classification of PDs along dimensional lines. A review of 21 taxometric studies of PDs and related variables calls an unqualified version of this view into question. Analyses of the three PDs investigated to date strongly support taxonic (i.e., categorical or discontinuous) models. Implications for the conceptualization and classification of PDs are drawn.

. Introduction Personality disorders (PDs) occupy an unusual position in psychiatric classification. Since introduction of DSM-III (American Psychiatric Association, 1980), they have been segregated from the great majority of mental disorders on a separate diagnostic axis. Their status as mental disorders has been questioned on many grounds, from pragmatic concerns about their diagnostic unreliability to more fundamental doubts about whether they represent diagnosable forms of abnormality. One reason for the marginal and controversial status of PDs is that they have two aspects—personality and disorder—about which the clinical community tends to hold contrasting views. Mental disorders, following the standard medical model, are often understood to be discrete pathological entities. Individuals are generally taken either to have or not to have a particular disorder. Degrees of severity may be acknowledged among the affected and subthreshold forms may be recognized, but having the disorder is nevertheless understood in a categorical fashion. Consistent with this view, DSM-IV (American Psychiatric Association, 1994) makes categorical diagnoses, often defining a cutoff point on a symptom checklist above which a disorder is judged to be present. Personality is often conceptualized in an entirely different manner. Traditionally, variations in personality are understood to be matters of degree rather than kind. Differential psychologists pioneered a view of traits as quantifiable dimensions and dismissed typological accounts of personality such as Jung's as coarse and prescientific. To this day, personality psychologists represent traits as graded continua and model their structure with statistical tools such as factor analysis that presume the adequacy of dimensional representations. The intellectual and historical context of this dimensional view of personality and of the categorical alternative are extensively discussed by Gangestad and Snyder (1985), Kagan (1994), and Meehl (1992). In the case of PDs these views collide. To the extent that a PD is a standard mental disorder it will be understood categorically, consistent with the default assumption embodied in DSM-IV. To the extent that a PD is a form of personality, however, a dimensional view seems appropriate, consistent with the representation of normal personality variation as continuous. Some of the controversies surrounding PDs are associated with these conflicting views. If PDs are grounded in normal personality continua, it seems questionable and arbitrary to conceptualize them as discrete categories, and the unreliability of their diagnosis may at least partly reflect this arbitrariness. The categorical versus dimensional status of PDs is not only an underlying basis for other controversies in the field, but has been a source of controversy in its own right since publication of DSM-III Frances, 1982 and Widiger & Clark, 2000. Many psychologists have argued for a dimensional view of PDs on a number of compelling grounds (e.g., Livesley et al., 1994 and Widiger & Costa, 1994). First, a dimensional view may be truer to the fundamental nature of PDs than a categorical view, so that DSM-IV's cutoffs impose arbitrary distinctions that misrepresent a seamless state of affairs. Second, by drawing arbitrary distinctions, the categorical view may reduce researchers' capacity to assess the correlates of PDs by weakening statistical power, and may lose severity-related information that is important for clinicians. Third, the categorical view is held responsible for the high degree of “comorbidity” among PDs, so that individuals who fall high on a dimension common to several PDs are spuriously represented as having several distinct conditions. In short, proponents of the dimensional view criticize the categorical alternative as crudely black-and-white in its structural assumptions, oversimplifying and falsely precise in its dichotomization, and uneconomical in its diagnostic application. Instead of being discrete categories, they argue, PDs represent extreme variants that fall on a continuum with normal personality. Advocates of the dimensional view can point to two main implications of their stance. First, the classification of PDs should be overhauled along dimensional lines, so that degrees of personality pathology can be assessed along a small number of nonredundant personality dimensions. Second, given the proposed absence of discontinuity between normal and abnormal personality, researchers can seek to understand PDs in terms of established models of normal personality. These two implications are obviously connected. If PDs were shown to be associated with particular trait dimensions, these dimensions might serve as bases for a radically revised dimensional classification of axis II. Attempts to understand PDs in terms of established dimensional models of normal personality are well advanced. The two most successful models used to date are the five-factor model (FFM; John & Srivastava, 1999) and the interpersonal circle (IC; Wiggins, 1982). Numerous studies have mapped PDs onto these models, with substantial success (e.g., Lynam & Widiger, 2001, Romney & Bynner, 1997, Sim & Romney, 1990, Soldz et al., 1993 and Trull, 1992). These literatures have been reviewed elsewhere Costa & Widiger, 1994 and Widiger & Hagemoser, 1997, and their rapid growth is evident in a recent special issue of the Journal of Personality (Ball, 2001) and a second edition of Costa and Widiger's (2001) volume on PDs and the FFM. Many PDs appear to be robustly correlated with the dimensions proposed by the respective dimensional models. At present, FFM, IC, and other dimensional systems of personality (e.g., Benjamin, 1996, Birtchnell & Shine, 2000 and Cloninger, 1987) are widely believed to offer not only a valuable description of at least some PDs, but also an understanding of what constitutes or underlies them. On the dimensional view, PDs are elevations on particular trait continua, rather than simply being empirically associated with them. The dimensional view of PDs has garnered considerable support among psychologists. The evidence supporting it has been described as “overwhelming” (Livesley, 1996, p. 224) and most interested psychologists probably adhere to it explicitly or implicitly. However, just as the categorical view of PDs embodied in DSM-IV lacks empirical support and largely represents a taxonomic assumption, the evidential support for the dimensional view is not as uniform or substantial as some have claimed. For instance, evidence that PD symptom distributions are phenotypically continuous, lacking clear breaks or bimodalities (Livesley, Jackson, & Schroeder, 1992), is not inconsistent with the existence of latent categories whose score distributions overlap. Evidence that PD symptom scores are associated continuously with functional impairment (Nakao et al., 1992) also does not rule out presence of latent categories, as category members (and nonmembers) may have varying degrees of symptom severity. The extensive evidence that normal personality dimensions have reliably patterned associations with PDs does not establish that these PDs are merely extreme variants on these dimensions. What is needed to adjudicate between categorical and dimensional views of PDs more definitively is a rigorous method for comparing their adequacy as structural models of particular PDs. The taxometric procedures developed by Paul Meehl and his colleagues (e.g., Meehl, 1995 and Waller & Meehl, 1998) are well suited to this task. Employed in roughly 70 empirical studies to date (see Haslam & Kim, in press, for a review), these procedures test between categorical (“taxonic”) and dimensional (“nontaxonic”) models by examining patterns of covariation among fallible indicators of the latent variable of interest. Maximum covariance analysis (MAXCOV), for example, examines the covariance of a pair of indicators as a function of a third. Such covariance should rise to a maximum where the latent distribution of category members intersects the distribution of nonmembers (i.e., where members and nonmembers are in equal proportions), but should be flat where no latent category exists. Means-above-minus-below-a-cut (MAMBAC), the second most popular taxometric procedure after MAXCOV, has a similar rationale. The mean value of one indicator is calculated for cases falling above and below a sliding cutpoint on a second indicator, and the difference between these means should also rise to a peak where members and nonmembers of a latent category have equal mixing proportions. Taxometric methods such as these rely heavily on plot inspection rather than null hypothesis significance testing, and on the convergent findings of multiple independent procedures (“consistency testing”). Extensive Monte Carlo evidence shows that they discriminate taxonic and nontaxonic data sets with high accuracy (e.g., Meehl & Yonce, 1994 and Meehl & Yonce, 1996), do so as well as or better than alternatives such as cluster analysis and mixture modeling (Cleland, Rothschild, & Haslam, 2000), and are reasonably robust under unfavorable measurement conditions such as skewed or dichotomous indicators (e.g., Haslam & Cleland, 1996 and Ruscio, 2000). Taxometric studies have become popular in abnormal psychology, and have recently examined such clinical phenomena as depression (Ruscio & Ruscio, 2000), dissociation (Waller, Putnam, & Carlson, 1996), eating disorders (Gleaves, Lowe, Snow, Green, & Murphy-Eberenz, 2000), and worry (Ruscio, Borkovec, & Ruscio, 2001). Livesley et al. (1994) noted that the taxometric methods employed in studies such as these “have the potential to advance theoretical understanding of the nature of … personality disorder” (p. 10), and cited three PD-related papers, but since then many more studies have appeared. The present paper reviews these studies to determine where the rapidly accumulating evidence stands. Does it tend to support the dimensional view of PDs, the categorical view, or neither? A comprehensive search for published and unpublished research, conducted using extensive Internet, PsycLIT and bibliographic searches, and personal inquiries to investigators, revealed 21 taxometric studies of PDs or related variables. Studies of the three PDs that have received taxometric scrutiny are reviewed below, followed by a discussion of their implications.

11. Conclusion Whether PDs should be classified dimensionally or categorically is a complex question with many facets. It is not simply a question of the structural validity of the alternative models, but also involves pragmatic considerations such as clinical convenience, parsimony, and predictive utility (Frances, 1982). As a result, taxometric evidence cannot resolve the question outright. Nevertheless, taxometric analyses offer particularly rigorous examinations of the structural basis of PDs that cannot be ignored by theorists and researchers interested in the issue of categorical versus dimensional diagnosis. Taxometric evidence favoring a categorical view of certain PDs needs to be taken into account in any forming consensus on this issue. Such evidence will need to be integrated with evidence favoring dimensionality, as neither alternative is likely to supersede the other completely. The taxometric research reviewed in this paper covers only three PDs and therefore does not allow any strong generalizations about the status of PDs as a set. The limited coverage of this research, and the heterogeneity of its measures, methods, and samples, sounds an urgent call for extensions and replications. However, the research that is now at hand disallows blanket claims that PDs are always and utterly dimensional. This position may prove to be accurate for some PDs, pending future work, but current evidence strongly suggests that it is inaccurate for several and that no firm expectations to the contrary are therefore justified for the rest. The categorical view of PDs cannot be completely dismissed at this point, and its tenability should be judged in part on the sort of empirical grounds that taxometric research provides. This view does not rule out the reality or predictive value of continuous variation in PD symptomatology, nor does it entail a crudely dichotomous understanding of PD categories. It can therefore be reconciled with some aspects of the dimensional view, especially where the supplementation of dichotomous diagnosis with quantitative assessment of severity is concerned. Generalized and unqualified theoretical positions on the dimensional versus categorical issue are unhelpful, and some kind of hybrid or pluralistic understanding of PDs is desirable.