Gene-Environment Interactions in Personality Disorders
Personality disorders are clinical syndromes in which personality traits cause psychopathology. Disorders are diagnosed when traits interfere with occupational and personal adjustment, leading to distress and/or dysfunction. Patients meeting these criteria develop abnormalities of behaviour, affect, and cognition that begin early in life, are pervasive in many contexts, and remain consistent over time.
The roots of personality disorders lie both in genetic vulnerability and environmental adversity. Research in every category of mental disorder demonstrates genetic predispositions associated with psychopathology (Paris, 1999). These vulnerabilities are then uncovered and unleashed by environmental stressors. Predisposition and stress have an interactive relationship: genetic variability influences the way individuals respond to their environment, while environmental factors determine whether genes are expressed.
These principles describe a general theory of the aetiology of mental disorders, the stress-diathesis model (Monroe and Simons, 1991; Paris, 1999). This model provides a frame for understanding the role of gene-environment interactions in personality disorders.
One of the main problems in identifying genetic and biological factors in personality disorders concerns how to define the phenotype (Jang et al., 2001). Diagnoses, as defined in the current psychiatric classification, are not very useful in this regard. Each of the present categories of personality disorders has some relationship to biological variables, but only to the extent that these disorders reflect traits. When we apply a wide range of research strategies (genetic associations, imaging, biological markers, or neuropsychological testing), relationships with measures of biological function are consistently stronger with traits, and weaker with disorders.
The relationship between traits and disorders is crucial to understanding the nature of personality pathology. Trait vulnerabilities, by themselves, do not explain why patients develop clinical symptoms. Instead, interactions between genetic and environmental factors, leading to pathological feedback loops, are responsible for the amplification of traits to dysfunctional levels.
AND PERSONALITY DISORDERS
Personality disorders, personality traits, and temperament have a hierarchical relationship (Rutter, 1987). Temperament refers to behavioural dispositions present at birth. Personality traits are individual differences in behaviour that remain stable over time and context. These characteristics represent an amalgam of inborn characteristics and the effects of life experiences. Personality disorders describe dysfunctional outcomes arising from traits.
In trait psychology, personality is measured as 'dimensions' with a normal distribution. Therefore, each personality profile should be common in the general population, and be compatible with normality. Disorders occur when these traits are amplified, and used in rigid and maladaptive ways.
Using current criteria, some studies (Weissman, 1993) have estimated that approximately 10% of the general population have a diagnosable personality disorder. Clearly, the precise figure depends on the cut-off point one uses. If personality is dimensional, one would expect to find no sharp break between traits and disorders. Research in clinical and community populations (Livesley and Jang, 2000) consistently supports this principle.
Personality disorders are even more common in treatment settings (Loranger et al., 1994). But they are not always recognized as such, since patients present clinically when they also have Axis I symptoms. Moreover, there are serious problems in the classification of personality disorders.
OF PERSONALITY DISORDERS
DSM-IV (American Psychiatric Association, 1994) divides personality disorders into 10 categories. ICD-10 (World Health Organization, 1992) uses a similar system of classification, describing most of the same types. DSM groups these diagnoses into three clusters (A, B, and C) that share common characteristics. A patient who meets the overall criteria for a disorder, but who does not fall into any specific category, is classified as 'personality disorder, not otherwise specified' (NOS). Many patients fail to fit prototypically into any single category, and about a third of all cases fall into the 'NOS' group (Loranger et al., 1994).
Cluster A, described as 'odd', includes schizoid, paranoid, and schizotypal disorders, all of which he in the 'schizophrenic spectrum' (Siever and Davis, 1991). Cluster B, which can be called either 'dramatic' or impulsive, includes antisocial, borderline, narcissistic, and histrionic disorders. Cluster C., described as 'anxious', includes avoidant, dependent, and compulsive disorders.
Many of mese categories exist largely on the basis of clinical tradition, and their validity is suspect. The constructs describing schizotypal, antisocial, and borderline personality are the most useful, since each of these diagnoses communicates crucial clinical information in a compact fashion. Thus, typical patients in each of these categories have a characteristic outcome and a characteristic treatment response.
One of the most serious difficulties with the existing categories is that they overlap, with most patients earning more than one diagnosis (Pfohl et al., 1986). Many (but not all) of these overlaps occur within clusters. The clusters therefore reflect underlying dimensions, which may be more valid than individual categories. In