The DSM-IV avoids the question by taking an empirical, atheoretical approach; the disorders are defined by descriptive criteria emphasizing observable behaviors. In the past, personality was traditionally viewed as a product of upbringing, whereas the major mental illnesses were thought to be related to biologic vulnerabilities. These issues are now understood to be much more complex, and a substantial body of evidence suggests that both biologic and environmental variables play important interacting roles in personality development and disorders.

A familial relationship may exist between schizophrenia and cluster A personality disorders, especially schizotypal personality disorder. Family studies also have suggested a hereditary component to antisocial personality disorder. Borderline personality disorder clusters in families, although this is not clearly genetically determined.

Some axis I illnesses such as depression are present at elevated rates in families of personality disordered probands, suggesting that in some cases personality disorder symptoms may be inherited subsyndromal forms of axis I problems.

Data supporting the role of environmental factors are strongest in the cluster B group, including high rates of childhood sexual and physical abuse as well as elevated rates of childhood stressors such as divorce, parental loss, inadequate parenting, frequent moves, and institutional placements. Although the association between borderline personality disorder and childhood abuse is the most strongly established (70–80% prevalence in most studies), other personality disorders have been estimated to have childhood abuse prevalence rates of approximately 50% compared with estimates of 20–40% in mixed psychiatric populations and 10–15% in the general population.