Short answers, pulled from the story.
A personality disorder is a mental disorder characterized by an enduring, pervasive maladaptive pattern of behavior, emotions, cognition, and inner experience that deviates from social norms. These patterns develop early, are inflexible, and cause significant distress or impairment in interpersonal relationships and self-functioning. A formal diagnosis requires the disturbances to be present for a minimum of two years.
A 2008 estimate based on six major studies across three nations put the median rate of diagnosable personality disorder in the general community at 10.6%, roughly one in ten people. A World Health Organization screening survey across 13 countries reported in 2009 a prevalence of around 6% using DSM-IV criteria. In the US, data from the National Comorbidity Survey Replication between 2001 and 2003 indicated a population prevalence of around 9%.
The DSM-5 uses a categorical approach, listing ten specific personality disorders grouped into three clusters (A, B, and C). The ICD-11 uses a dimensional approach, classifying a single unified personality disorder as mild, moderate, or severe based on the degree of impairment in self-functioning and interpersonal relationships. The DSM-5 also includes an alternative dimensional-categorical hybrid model in its Section III, called the AMPD.
Personality disorders result from a combination of genetic, environmental, and neurobiological factors. Childhood trauma, abuse, neglect, and adverse parenting are established risk factors. A study of 793 mothers and children found that verbal abuse made children three times more likely to develop certain personality disorders in adulthood. Neurobiological research has found structural brain differences in people with personality disorders, including a hippocampus that can be up to 18% smaller than average.
Psychotherapy is the primary treatment. Dialectical behavior therapy (DBT) is recommended by NICE for borderline personality disorder, and cognitive behavioral therapy has evidence for avoidant, obsessive-compulsive, and dependent types. No medication has been approved by the U.S. Food and Drug Administration specifically for treating any personality disorder, and the evidence for pharmaceutical benefit is weak across most types.
A key challenge is egosyntonicity: many people with a personality disorder do not experience their personality as disordered and therefore do not seek treatment. A study of 68 patients on an assertive community team found a three-to-one ratio of treatment-resistant to treatment-seeking cases. High comorbidity with other mental disorders, the enduring nature of the patterns, and the difficulty of building a stable therapeutic alliance also contribute to the challenges of treatment.