Relatively little is known about the causal factors involved in the development of most personality disorders. One reason for this is that personality disorders only began to receive consistent attention from researchers after they entered the DSM in 1980. Another problem stems from the high level of comorbidity among them. For example, in an early review of four studies, Widiger and colleagues found that 85 percent of patients who qualified for one personality disorder diagnosis also qualified for at least one more, and many qualified for several more (Widiger & Rogers, 1989 ; Widiger et al., 1991 ). A study of nearly 900 psychiatric outpatients reported that 45 percent qualified for at least one personality disorder diagnosis and, among those with one, 60 percent had more than one and 25 percent had two or more (Zimmerman et al., 2005 ). Even in a nonpatient sample, Zimmerman and Coryell ( 1989 ) found that of those with one personality disorder, almost 25 percent had at least one more (see also Mattia & Zimmerman, 2001 ; Trull et al., 2012 ). This substantial comorbidity adds to the difficulty of untangling which causal factors are associated with which personality disorder.
Another problem in drawing conclusions about causes occurs because researchers have more confidence in prospective studies, in which groups of people are observed before a disorder appears and are followed over a period of time to see which individuals develop problems and what causal factors have been present. Although this has begun to change, to date, relatively little prospective research has been conducted with most of the personality disorders. Instead, the vast majority of research has been conducted on people who already have the disorders; some of it relies on retrospective recall of prior events, and some of it relies on observing current biological, cognitive, emotional, and interpersonal functioning. Thus, any conclusions about causes that are suggested must be considered very tentative.
Of possible biological factors, it has been suggested that infants’ temperament (an inborn disposition to react affectively to environmental stimuli; see Chapter 3 ) may predispose them to the development of particular personality traits and disorders (e.g., L. A. Clark, 2005 ; Mervielde et al., 2005 ; Paris, 2012 ). Some of the most important dimensions of temperament are negative emotionality, sociability versus social inhibition or shyness, and activity level. One way of thinking about temperament is that it lays the early foundation for the development of the adult personality, but it is not the sole determinant of adult personality. Given that most temperamental and personality traits have been found to be moderately heritable (e.g., Bouchard & Loehlin, 2001 ; Livesley, 2005 ), it is not surprising that there is increasing evidence for genetic contributions to certain personality disorders (e.g., Kendler et al., 2008 , 2011 ; Livesley, 2005 , 2008 ; Livesley & Jang, 2008 ; South et al., 2012 ; Torgersen et al., 2000 ). However, for at least most disorders, the genetic contribution appears to be mediated by the genetic contributions to the primary trait dimensions most implicated in each disorder rather than to the disorders themselves (Livesley, 2005 ; Kendler et al., 2008 ). In addition, some progress is being made in understanding the psychobiological substrate of at least some of the traits prominently involved in the personality disorders (e.g., Depue, 2009 ; Depue & Lenzenweger, 2001 , 2006 ; Livesley, 2008 ; Paris, 2005 , 2007 ; Roussos & Siever, 2012 ).
Genetic propensities and temperament may be important predisposing factors for the development of particular personality traits and disorders. Parental influences, including emotional, physical, and sexual abuse, may also play a big role in the development of personality disorders.
Among psychological factors, psychodynamic theorists originally attributed great importance in the development of character disorders to an infant’s getting excessive versus insufficient gratification of his or her impulses in the first few years of life (Fonaghy & Luyten, 2012). More recently, learning-based habit patterns and maladaptive cognitive styles have received more attention as possible causal factors (e.g., Beck et al., 1990 , 2004 ; Lobbestad & Arntz, 2012 ). Many of these maladaptive habits and cognitive styles that have been hypothesized to play important roles for certain disorders may originate in disturbed parent–child attachment relationships rather than derive simply from differences in temperament (e.g., Benjamin, 2005 ; Fraley & Shaver, 2008 ; Meyer & Pilkonis, 2005 ; Shiner, 2009 ). Parental psychopathology and ineffective parenting practices have also been implicated in certain disorders (e.g., Farrington, 2006 ; Paris, 2001 , 2007 ). Many studies have also suggested that early emotional, physical, and sexual abuse may be important factors in a subset of cases for several different personality disorders (Battle et al., 2004 ; Grover et al., 2007 ).
Various kinds of social stressors, societal changes, and cultural values have also been implicated as sociocultural causal factors (Paris, 2001 ). Ultimately, of course, the goal is to achieve a biopsychosocial perspective on the origins of each personality disorder, but today we are far from reaching that goal.