Dimensional models of personality disorders (also known as the dimensional approach to personality disorders, dimensional classification, and dimensional assessments) conceptualize personality disorders as quantitatively—not qualitatively—distinct from normal personality, viewing disorders as extreme positions on continuous trait dimensions shared by the general population. They consist of extreme, maladaptive levels of certain personality characteristics – commonly described as facets within broader personality factors or traits. This is contrasted with the categorical approach, such as the standard model of classification in the DSM-5.
Within the context of personality psychology, a "dimension" refers to a continuum on which an individual can have various levels of a characteristic, in contrast to the dichotomous categorical approach in which an individual does or does not possess a characteristic. In regards to personality disorders, this means that they are classified according to which characteristics are expressed at which levels. This stands in contrast to the traditional categorical models of classification, which are based on the boolean presence or absence of symptoms and do not take into account levels of expression of a characteristic or the presence of any underlying dimension.
The way in which these diagnostic dimensions should be constructed has been under debate, particularly in the run up to the publication of the DSM-5. A number of dimensional models have been produced, differing in the way in which they are constructed and the way in which they are intended to be interpreted. There are four broad types of dimensional representation, although others also exist: Dimensional representation of the original DSM categories of personality disorders; Dimensional representation based on identification of latent traits with the DSM disorders; Dimensional representation based on the traits from normal personality research; Representation based on integration of dimensional modals, e.g. by using network analysis.
The dimensional approach is included in Section III ("Emerging Measures and Models") of the fifth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5), where it is titled Alternative DSM-5 Model for Personality Disorders. The decision to retain the old DSM-IV categorical model of personality disorders in DSM-5 was controversial, and efforts continue to persuade the American Psychiatric Association to replace it with the dimensional model in DSM 5.1.
Usage Dimensional models are intended to reflect what constitutes personality disorder symptomology according to a spectrum, rather than in a dichotomous way. As a result of this they have been used in three key ways; firstly to try to generate more accurate clinical diagnoses, secondly to develop more effective treatments and thirdly to determine the underlying etiology of disorders.
Clinical diagnosis The "checklist" of symptoms that is currently used is often criticized for a lack of empirical support and its inability to recognize personality-related issues that do not fit within the current personality disorder constructs or DSM criteria. It has also been criticized for leading to diagnoses that are not stable over time, have poor cross-rater agreement and high comorbidity suggesting that they do not reflect distinct disorders. In contrast the dimensional approach has been shown to predict and reflect current diagnostic criteria, but also add to them. It has been argued to be especially useful in explaining comorbidity which is often high for patients diagnosed with a personality disorder. Following from these claims, the fifth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5) incorporates a combined categorical-dimensional approach to diagnosing personality disorders based on the degree to which a person shows elevated levels of particular personality characteristics. However, one of the issues in using a dimensional approach to diagnosis has been determining appropriate cut off points so as to know who belongs to the category of people requiring treatment; this is partly why both categorical and dimensional diagnoses are included.
Since the categorical model is widely used in clinical practice and has a significant body of research supporting it, its common usage is compelling to laypeople when they are judging the credibility of professional opinion. Therefore, the dimensional approach is often further criticized for being difficult to interpret and less accessible. It is however widely used in some professional settings as the established approach, for example by forensic psychologists.
Treatment effectiveness Another suggested usage of the dimensional approach is that it can aid clinicians in developing treatment plans and assessing other mechanisms contributing to patient's difficulty in functioning within the social, personal, or occupational domains. The approach can improve treatment in two ways. Firstly it can enable development of more personalized care plans for individuals based on their adaptive and maladaptive characteristics. Secondly, it means that relevant symptomology which is not considered maladaptive can be considered when developing and evaluating general therapeutic and medical treatment.
Determining cause Attempts at presenting an etiological description of personality disorders have been avoided due to the influence of the DSM and its principles in psychiatric research (See history section). However some techniques are looking at potential interrelated causalities between symptoms of personality disorders and broader influences including aspects of normal personality (See integrated approaches section).
History Initial development of a categorical model The adoption of a categorical approach to personality disorders can be understood in part due to ethical principles within psychiatry. The 'do no harm principle' led to Kraepelinian assumptions about mental illness and an emphasis on empirically grounded taxonomic systems that were not biased by unsubstantiated theories about etiology. A taxonomic checklist based on empirical observations rather than bias prone theoretical assumptions developed. It was both categorical and hierarchical, with the diagnosis of a disorder being dependent on the presence of a threshold number categories (usually five) out of a total number (seven to nine) Disorders were organized into three clusters, existing purely to make the disorders easier to remember by associating them with others that have similar symptoms, not based on any theory about their relatedness.
Emerging problems with the categorical model The dimensional model was developed in response to the limitations of this standard categorical model. The expectations from a Kraepelinian approach were that as systematic research into psychiatric health increased; diagnostic categories would be refined and targeted reliable treatments would be developed. However this reductionist approach to diagnostic categorization has led to disorders with high comorbidity, life course instability, poor treatment effectiveness and poor diagnostic agreement. In addition the findings from psychopathological research have led to an increasing body of evidence suggesting overlaps between normal and maladaptive personality and interrelatedness across disorders. These findings have been further supported by genetic and developmental studies which have constantly pointed towards greater interrelatedness then the diagnostic categories can offer. These consistently disconformity findings, alongside the successful shift to a continuous rather than categorical approach in other areas of research, such as regarding ASD, led to consideration of alternative approaches.
Development of methodological techniques Factor analysis The development of factor analysis as a popular statistical technique in differential psychology has led to an increase in attempts at finding underlying traits. More recently this has been used in the context of personality disorders both as a means of looking at which personality traits current categorical diagnoses are related to and also as a method of looking for new psychopathological latent variables. Factor analysis has helped illustrate that the full range of relevant personality pathology is not included in the DSM psychiatry nosology. However the technique does not show information about a continuum from normal to clinically relevant personality.
Dimensional analysis Dimensional classification techniques show individual multidimensional profiles and therefore they can show information about a personality continuum (from normal to atypical), one such technique is Hybrid modeling.Helzer, J. E., Kraemer, H. C., Krueger, R. F., Wittchen, H. U., Sirovatka, P. J., & Regier, D. A. (Eds.). (2009). Dimensional approaches in diagnostic classification: Refining the research agenda for DSM-V. American Psychiatric Pub. Cut off points can be introduced into these modals to show where a diagnosis may lie. However the number of different rating scales that need to be looked at and the lack of interdisciplinary research between statisticians and psychologists has meant that attempts at finding a 'worldwide' criteria for dimensional diagnosis using this method has been of limited success.
Comparative analysis Analyses have been conducted to test the relative fit of categorical and dimensional modals to evaluate whether single diagnostic categories are suited to either status. These types of analysis can include a range of data, including endophenotypes or other genetic or biological markers which increases their utility. Multivariate genetic analysis helps establish how well the current phenotypically developed structure of personality disorder diagnosis fits with the genetic structure underlying personality disorders. Results from these types of analysis support dimensional over categorical approaches.
Network analysis Network Analysis has been used as a means of integrating information about personality with personality disorders and as well as information about other genetic, biological and environmental influences into a single system and looking at interrelated causalities between them (See integrated modals).
Model development Adapted categorical models There are different ways to 'dimensionalize' categorical personality disorders; these can be summarised into two categories.
The first involves quantifying DSM-5 pathology. This can be done either based on the degree to which symptoms are present or on how close to a prototypic presentation a patient's presentation may be. The prototype approach includes features not present in the DSM. The second approach involves identification of DSM disorder traits by means of factor analysis to show underlying dimensions of the personality disorder criteria, this method may also include relevant psychopathology.
Normal personality models Five-factor model
The Five-Factor model of personality, which is the most dominant dimensional model, has been used to conceptualize personality disorders and has received various empirical support. Under this approach, extreme levels of the basic personality traits identified by the FFM are what contributes to the maladaptive nature of personality disorders. Over 50 published studies supporting this model have been identified, providing much empirical support for this approach. Most of these studies examine the relationship between scores on separate measures of Big Five trait and personality disorder symptoms.