Delusions are firmly held, yet clearly erroneous, beliefs that tend to indicate that the person is suffering from some medical, neurological or mental impairment, rather than simply not being accustomed to a particular culture, religion or level of intelligence.
Delusions can be symptomatic of a variety of mental disorders, including anorexia, psychosis, schizophrenia, bipolar disorder, major depressive disorder and dementia. Some psychoactive drugs can also cause delusional thinking. Delusions differ from misconceptions both in the degree to which the person thinks the belief is true and due to the fact that the delusion will be clung to, despite clear evidence to the contrary.
Delusions can be either bizarre (aliens among us) or non-bizarre (one that is not true but actually could be, such as being the subject of surveillance). Mood congruent delusions are consistent with a particular state. For example, a person suffering from depression may think the world is over while a manic person may think she is famous. Mood incongruent delusions are either not consistent with the present mood or are mood neutral (for example, a depressed person who thinks someone is controlling his mind).
Most delusions fall into a common theme, such as control, nihilism, jealousy, guilty, mind reading, reference, erotomania, grandiosity, persecution, religion, and somatic. For example, someone with cocaine-induced delusions may think bugs are crawling up his arm (parasitosis) or may see flashing lights that really aren’t there.