It is first necessary to discuss how 'normal' personality may be classified. There are two general approaches, the ideographic and the nomothetic.

Ideographic theory views each person as a unique individual, with aspects of personality that are not possessed by anyone else. The structure of the personality may be different between individuals, even though similar traits may be present; the relative importance of such traits may also differ. This approach gives great weight to detailed appreciation of the individual history in case studies.

Nomothetic theory, by contrast, sees each person as possessing greater or lesser amounts of a number of personality traits, these traits being present to a greater or lesser extent in all members of the population. In other words, each personality, according to nomothetic theory, is made up of a unique selection from a sort of a la carte menu of personality traits, each of which is assumed to have the same meaning or effect in each person.

The nomothetic model has been predominant in recent years, as it is obviously suitable for a quantitative approach using self-report personality questionnaires, structured interviews, and factor analysis of the results thereof. For example, it has been suggested that a five-factor model (neuroticism, extroversion, openness, agreeableness, and conscientiousness) can account for much of the observed personality characteristics of different individuals, and that there may be a strong genetic aspect to such traits (Yamagata et al., 2006).

Although the evidence base for a nomothetic approach to understanding personality may continue to grow, it is counter-intuitive to think that it will ever be able to account for the infinite variety of individual personality.

Use of personality measures such as the MMPI (Minnesota Multiphasic Personality Inventory) is claimed to give quantitative measures of aspects of personality. However, their use remains uncommon in UK clinical practice in psychiatry.

The various types of personality disorder, as described below, overlap with each other, many individuals having traits characteristic of more than one type. Hence, if a patient satisfies criteria for more than one type, it does not necessarily mean that he has 'two personality disorders', in the sense of two separate conditions that are additive. The disorders are therefore not necessarily 'twice as severe' - it is more likely that they just do not fit neatly into the written classifications.

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