Kendell (Reference Kendell2002) refers to treatment factors influencing attitudes; I believe that level of understanding and the concept of mental illness are additional influences. Mental disorders can be defined as abnormalities of higher mental function (i.e. perception, thought, emotion, memory) and can be explained in relation to different degrees of reductionism, which is consistent with the application of a medical model. Some have evidence of biological dysfunction (e.g. epilepsy), others of psychological dysfunction (e.g. schizophrenia) and others of ‘behavioural dysfunction’ (e.g. personality disorders). The confidence in what constitutes a mental disorder is partly dependent on the level of explanatory power. Behaviour lacks explanatory power because of the difficulty in determining whether behaviours arise from normal or abnormal brain function. In this context operational definitions reliant on behaviour are based on poorer-quality evidence and more heavily influenced by sociocultural factors.

In addition, eliciting symptoms of mental illness relies on subjective reports of the experiential aspects of internal mental processes. A mental illness is recognised when qualities of internal mental experiences that are recognised as being different from normal mental experiences are reported, and the larger the difference the more likely it will be explained as mental illness. This phenomenon is usually lacking in people with personality disorders.

Thus, personality disorders lack good-quality evidence of altered higher mental function, including internal mental experiences, and using the definition above they would not be considered mental disorders. People with personality disorder experience disadvantage in their sociopolitical environment, often due to their behaviour, and in other circumstances ‘disadvantage’ has been sufficient to explain increased morbidity. Until altered higher mental function can be reliably demonstrated it may not be appropriate to view personality disorders as mental disorders. The term ‘challenging behaviour’ is used in the psychiatry of learning disability and has the advantage of being descriptive, making no assumptions about aetiology, and is more explicit about a social dimension; it may be possible to develop a similar term for personality disorders. This does not obviate the need for management of personality disorders but clarifies the concept of what a mental disorder is.