F24 — Induced delusional disorder

What is this category?

F24 describes the rare situation where two (occasionally more) people who share an unusually close relationship, and who are often socially isolated together, come to share the same or a closely related delusional belief. One person — the primary case — has an established psychotic or delusional disorder (often F20 or F22) and is usually the more dominant or intellectually influential partner; the other person — the induced partner — adopts the belief mainly because of the relationship, not because they have an independent psychotic illness of their own.

Core diagnostic concept

The classification question F24 answers is: "has this person adopted a delusional belief mainly because of a close relationship with someone else who already holds it, rather than developing it independently?" F24 is fundamentally a relational classification: it cannot be assessed by looking at the induced person's belief in isolation — the relationship, the primary case, and the order in which the belief appeared are all part of the picture.

Key diagnostic features

A close, often socially isolated relationship

The two people are usually family members (frequently sisters, or a parent and adult child, or spouses) who live together and have limited contact with the outside world.

A primary case with an established disorder

One person already has a psychotic or persistent delusional disorder (most often F20 or F22) and is typically the more dominant, articulate, or intellectually influential partner in the relationship.

An induced partner without an independent psychotic history

The second person adopts the same, or a very closely related, delusional belief, usually without an independent history of psychotic illness before the relationship, and often with fewer psychotic features overall than the primary case.

Belief content closely mirrors the primary case

The induced belief is not simply similar in theme — it typically matches the primary case's delusion closely in content and timing, having appeared after the primary case's belief was established.

Fading on separation

Separating the induced person from the primary case often leads to at least partial, sometimes complete, fading of the induced belief — a distinctive feature not expected in an independently arising delusion.

How to weigh evidence

These learning labels are teaching aids, not official ICD-10 terminology (see Sources and Method).

• supports A belief that closely mirrors, in content and timing, one already held by a close relative with an established delusional disorder, in a socially isolated pair.
• consistent not decisive The induced person describes the belief with genuine conviction, not merely humouring the primary case.
• raises alternative The 'induced' person actually developed the belief independently, before meeting or living with the primary case.
• contradicts or weakens The belief persists unchanged, or even strengthens, after a substantial period of separation from the primary case.
• missing and necessary to ask Who held the belief first, and how socially isolated the pair or group actually is.

Duration and course

Minimum duration: No separate minimum-duration threshold is specified for F24 itself; what matters most is establishing the relational origin of the belief, not how long it has lasted.

Course: Course often follows the relationship: the induced belief may persist as long as the close relationship and shared isolation continue, and often fades, partially or fully, if the pair is separated.

Included presentations

• A person living in a close, socially isolated relationship with someone who has an established delusional or psychotic disorder, who comes to share the same or closely related delusional belief.
• The same pattern occurring within a small family or household group rather than strictly two people (sometimes called folie à plusieurs).

Exclusions and distinguishing presentations

Common learner errors

• Common mistake: Assessing a shared delusion in isolation, without asking about the relationship

Worked example — a belief that arrived with a relationship

Nadia, 33, has lived alone with her older sister Yasmin for the past three years, the two having become increasingly socially isolated from friends and other family over that time. Yasmin has for two years held a firm, detailed belief that a neighbourhood group is monitoring the sisters through their electricity meter, and has recently begun making the same claim herself, in very similar terms to Yasmin's. Nadia has no prior history of unusual beliefs, no hallucinations, and no odd speech or behaviour outside this one topic. When a caseworker temporarily arranges for Nadia to stay with a cousin for two weeks, her conviction noticeably softens.

1. Step 1 — Identify the relationship: the sisters are closely bonded and socially isolated together, the classic setting for induced delusional disorder.
2. Step 2 — Identify the primary case: Yasmin held the belief first, for longer, and in more elaborated detail — she is the primary case.
3. Step 3 — Check the induced partner's independent history: Nadia has no prior history of unusual beliefs and no broader psychotic features of her own.
4. Step 4 — Check the response to separation: Nadia's conviction softens when temporarily separated from Yasmin, a distinctive F24 feature.
5. Conclusion — F24 is best supported for Nadia: a delusion adopted mainly through a close relationship with an established primary case, softening on separation.

Interactive practice

• Type A A belief that arrived with a relationship Recognition / application · Foundation
• Type B A shared belief — but is it only shared? Boundary / differential · Boundary
• Type C Two people, one belief, unclear order Insufficient information · Developing
• Type D A belief older than the relationship Progressive revision · Transfer

Retrieval prompts

Try answering each of these from memory before revealing where to check. Nothing you think or type here is sent or saved anywhere.

1. What single relational question is most decisive in identifying F24?

   Reveal answer location

   Check Duration and course, Diagnostic boundary, or the rapid revision card below — the cue you need is one of those, in this category's own already-authored wording.

   I recalled this I need to review this

2. Why can F24 not be assessed by looking at the induced person's belief in isolation?

   Reveal answer location

   Check Duration and course, Diagnostic boundary, or the rapid revision card below — the cue you need is one of those, in this category's own already-authored wording.

   I recalled this I need to review this

3. What commonly happens to the induced belief on separation, and why is this useful evidence?

   Reveal answer location

   Check Duration and course, Diagnostic boundary, or the rapid revision card below — the cue you need is one of those, in this category's own already-authored wording.

   I recalled this I need to review this

These self-checks are a personal study aid only, visible to you on this page for this visit — they are not stored and do not affect your mastery dimensions.

Related glossary terms

• Delusion

  A fixed, false belief that is firmly held despite clear evidence to the contrary, and that is not accounted for by the person's cultural or religious background.

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• Insight (clinical)

  The degree to which a person recognises that their experiences (such as unusual beliefs or perceptions) may reflect an illness rather than external reality. Insight can vary and is not itself a classification criterion, but is often noted educationally.

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