Tahir Rahman, MD

[Overvalued ideas are] a passionate attitude, also known as ‘fanatic’ in lay terms. One important aspect of overvalued ideas is that they are shared with other people, making them potentially destructive. Remember that delusions, by contrast, are uniquely false ideas held by individuals and identified by others as erroneous. While most people would not jeopardize their careers or lives for overvalued ideas, some will (and are secretly regarded as heroes by those less inclined to fight for an idea). (Oliver Freudenreich, Harvard Medical School)

An overvalued idea is an isolated preoccupying belief, neither delusional nor obsessional in nature, which comes to dominate a person’s life for many years and may affect his actions. The preoccupying belief may be understandable when the person’s background is known. (Gelder, Gath, Mayou, Cohen: Oxford Textbook of Psychiatry)

An overvalued idea is a thought shared by others in a society or culture but in the patient held with an intense emotional commitment capable of provoking dominating behavior in its service. An overvalued idea differs from a delusion in that delusions are false ideas unique to the possessor, whereas overvalued ideas develop from assumptions and beliefs shared by many others. An overvalued idea differs, too, from an obsession in that, although it dominates the mind as an obsession does, the subject does not fight an overvalued idea but instead relishes, amplifies, and defends it. Indeed, the idea fulminates in the mind of the subject, growing more dominant and more resistant to challenge. (Paul R. McHugh, Johns Hopkins Hospital)

[The definitive test of a delusion is] whether an unusual belief is shared by members of the patient’s subculture. Delusions must also be differentiated from overvalued ideas, which are fanatically maintained notions, such as the superiority of one sex, nation, or race over others . . . (George Winokur, Iowa and Paula Clayton, Washington University)

The background on which an overvalued idea is held is not necessarily unreasonable or false. It becomes so dominant that all other ideas are secondary and relate to it: the patient’s whole life comes to dominate around this one idea. It is usually associated with a very strong affect that the person because of his temperament has great difficulty in expressing . . . (Oyebode, Sims Symptoms in the Mind, London)