Obsession vs Overvalued Idea vs Delusion

Overview

All three can dominate a person's thinking, but they sit at three different points on a spectrum of conviction and insight. An obsession is recognised as one's own but unwelcome and resisted; an overvalued idea is held with strong conviction and some openness to challenge; a delusion is held with fixed, unshakeable certainty that no counter-evidence can touch. Insight — how the person themselves relates to the thought — is the axis that separates all three, not how strange or false the content sounds.

Dimension	obsession	overvalued-idea	delusion
Is the thought recognised as the person's own?	Yes — the person knows the thought originates from within themselves, however unwelcome	Yes, without question — there is no sense of the idea being foreign or externally caused	Yes, in the sense that it is not attributed to an outside agency (unlike thought insertion) — but the belief itself is held as objectively, externally true
Is it resisted?	Yes — the person actively tries to dismiss, suppress, or resist it, and this resistance is part of what defines it as obsessional	Not typically — the person does not experience the idea as something to be resisted, only as something that dominates their thinking	No — there is no internal resistance, since the person does not experience the belief as a problem to be pushed away
Degree of conviction	Low to none in the content itself — the person usually knows the thought is excessive or irrational even while unable to stop it	High, and sustained over a long period, though not absolute	Complete and fixed — held with a certainty essentially impervious to argument
Openness to counter-evidence	Not applicable in the same way — the person already agrees the thought is unreasonable; the difficulty is stopping it, not believing it	Some — more openness to discussion and outside perspective than a delusion, even if the idea rarely actually shifts	None — essentially unresponsive to reasoned argument or contrary evidence
Plausibility of content	Content is often exactly what the person would least want to think, and is recognised by them as excessive or absurd	Usually has some genuine basis in reality, even if pursued to an extreme or disproportionate degree	Often implausible or impossible, though plausibility alone does not define it — a delusion can occasionally turn out to be factually true
Typical example	A new parent is tormented by unwanted, intrusive thoughts of harming their baby, finds the thoughts horrifying, and tries hard, without success, to push them away	A person becomes consumed for years by the conviction that a past workplace grievance was a grave injustice, dominating their conversation and decisions long after most people would have moved on	A person is completely and unshakeably certain that a neighbour is transmitting radio signals into their home, and no explanation or evidence changes this belief

Common confusion

Students often try to sort these three by how bizarre or extreme the content sounds, assuming the strangest-sounding belief must be the delusion. Content is a poor guide: an overvalued idea can be about something entirely ordinary (a grievance, a diet, a cause) pursued to an extreme degree, while some delusions have mundane-sounding content. The reliable axis is the person's own relationship to the thought — resisted and recognised as irrational (obsession), dominant but somewhat open to challenge (overvalued idea), or fixed and unshakeable (delusion).

Clarifying questions

Do you feel that this thought is something you're trying to fight off or push away, or is it just something you believe and think about a great deal?
If someone gave you good evidence against this belief, do you think it would change your mind at all, even a little?
Looking at this belief now, does it seem excessive or unreasonable to you, even if you still think or worry about it?

Provenance

Class B · tier T1 · lifecycle migrated_candidate

Casey, P. & Kelly, B. (2019). Fish's Clinical Psychopathology, 4th ed. Ch. 4, 'Disorders of Thought and Speech'.