In the DSM-IV (Glossary) delusions were defined as follows:
Delusion. A false belief based on incorrect inference about external reality that is firmly sustained despite what almost everyone else believes and despite what constitutes incontrovertible and obvious proof or evidence to the contrary. The belief is not one ordinarily accepted by other members of the person's culture or subculture (e.g., it is not an article of religious faith). When a false belief involves a value judgment, it is regarded as a delusion only when the judgment is so extreme as to defy credibility.
This is how delusions are described in the DSM-5 (Schizophrenia Spectrum and Other Psychotic Disorders):
Delusions are fixed beliefs that are not amenable to change in light of conflicting evidence. Their content may include a variety of themes (e.g. persecutory, referential, somatic, religious, grandiose).[…] Delusions are deemed bizarre if they are clearly implausible and not understandable to same-culture peers and do not derive from ordinary life experiences. […] The distinction between a delusion and a strongly held idea is sometimes difficult to make and depends in part on the degree of conviction with which the belief is held despite clear or reasonable contradictory evidence regarding its veracity.
Although differences are not very significant, I do welcome some of the noticeable shifts.
First, as many philosophers and psychologists have noticed when commenting on the DSM-IV definition (e.g., Davies et al. 2001, pp. 133-134; Coltheart 2007, p. 1043), delusions need not be false, and being false is no longer a necessary condition for a belief to be delusional in the DSM-5 description.
Second, delusions do not need to be about external reality or to be based on incorrect inference. They could be about oneself and one’s own experiences, requiring little or no inference.
Third, we may have no proof against the truth of a belief, even when the belief is wildly implausible, and this is reflected in the move from the phrase “despite what constitutes incontrovertible and obvious proof or evidence to the contrary” to the phrase “despite clear or reasonable contradictory evidence regarding its veracity”.
More important, in my eyes the new account narrows the gap between delusions and other irrational beliefs, suggesting that the epistemic features of delusions are not unique to pathologies of the mind, but characterise many of our everyday beliefs.
One change I would have welcomed is the recognition that, although delusions are very resistant to counter-evidence and counter-argument, they are not totally impervious to cognitive probing. There are some interesting case studies suggesting that people can give up their delusion after being invited to reason about the inconsistencies that the delusion is introducing in their belief systems (e.g., McKay & Cipolotti 2007).
Something to consider for DSM-6...