Dimensions of personality describe variations in behavioural dispositions in the population. Because most personality dimensions are normally distributed, it is easy to assume that ‘abnormality’ lies at the tails of the distributions. For example, people with very high neuroticism scores might suffer much from anxiety, and those with very high conscientiousness scores might be disablingly rigid. However, might there be qualitative, as opposed to statistical, differences between the bulk of the population and a few individuals with very unusual personalities? The concept of personality disorder or abnormal personality lies within the domain of psychiatry and clinical psychology. Strangely, although it alludes to human personality variation, until the 1990s personality disorder attracted relatively little interest from differential psychologists studying normal personality. Conversely, the largely medically oriented researchers in the field of personality disorders have until recently shown little interest in either normal personality dimensions or the techniques of differential psychology.

It is commonplace for books on personality to make little or no mention of the research on personality disorders, let alone try to integrate normal and abnormal personality research, though there are exceptions (Larsen and Buss, 2006).