Category · F90-F98 block
F91 — Conduct disorders
What is this category?
F91 covers a repetitive and persistent pattern of dissocial, aggressive, or defiant conduct, which, when at its most extreme, amounts to major violations of age-appropriate social expectations and is therefore more severe than ordinary childish mischief or adolescent rebelliousness. A single dissocial or criminal act is not, by itself, grounds for this classification.
Core diagnostic concept
F91 requires a persistent pattern (typically judged over 6 months or more) of behaviour such as excessive fighting or bullying, cruelty to people or animals, severe destructiveness, fire-setting, stealing, repeated lying, truancy, or severe and persistent disobedience — described as a behavioural pattern, never as a judgment on the child's character or worth. The fourth character then records the setting (confined to the family versus extending into peer and community life) and the child's peer relationships (unsocialized versus socialized).
Key diagnostic features
- Persistent pattern, not an isolated act
- A repetitive, ongoing pattern of dissocial, aggressive, or defiant behaviour, not a single incident, however serious.
- Beyond ordinary rebelliousness
- The behaviour is more severe and persistent than the mischief or defiance seen in typical childhood or adolescent development.
- Setting
- Whether the pattern is confined to the family context (F91.0) or extends into peer and community settings (F91.1/F91.2).
- Peer relationships
- Whether the child is generally isolated, rejected, or unpopular with peers (unsocialized, F91.1) or well integrated into a peer group, such as a gang (socialized, F91.2).
How to weigh evidence
These learning labels are teaching aids, not official ICD-10 terminology (see Sources and Method).
- supports Over the past year, a 13-year-old has repeatedly been physically cruel to a family pet, initiated fights at school, stolen from classmates on multiple occasions, and been truant for weeks at a time.
- consistent not decisive A teenager 'gets into trouble a lot' with no further detail on frequency, severity, or duration.
- raises alternative A single episode of shoplifting, otherwise isolated, with no other pattern of aggressive, destructive, or rule-violating behaviour before or since.
- contradicts or weakens The behaviour occurs only in direct response to a specific, time-limited family crisis and resolves once the crisis passes, with no pattern before or after.
- missing and necessary to ask Whether the behaviour is confined to the home or also occurs at school and in the community, and what the young person's peer relationships are like.
Diagnostic boundary
F91 is distinguished from F90 by which feature is primary — rule-violating, aggressive conduct in F91 versus attention/activity regulation difficulty in F90, though the two can co-occur (F90.1). It is distinguished from F92 by the absence (in F91) of a genuine, persistent, co-occurring emotional disturbance meeting its own threshold. A single antisocial or criminal act, however serious, does not by itself meet F91 without a persistent pattern.
Full comparisons:
Duration and course
Minimum duration: A persistent pattern is required, typically judged over a period of 6 months or more, not a single incident.
Course: Pattern and severity can change with age, intervention, and changes in the young person's environment and relationships.
Included presentations
- A persistent pattern of dissocial, aggressive, or defiant conduct exceeding ordinary childhood mischief or adolescent rebelliousness, its setting and the young person's peer relationships determining the fourth character.
Exclusions and distinguishing presentations
| If this is true instead… | Consider | Why |
|---|---|---|
| A single dissocial or criminal act, without a broader persistent pattern | No specific code — see reasoning | F91 requires a persistent pattern; an isolated act, however serious, does not by itself meet this threshold. |
| Defiant, rule-violating behaviour together with a co-occurring hyperkinetic disorder meeting its own criteria | F90 | When both patterns are independently present, F90.1 (hyperkinetic conduct disorder) captures the combination; F91 alone is used when no hyperkinetic pattern is also present. |
Four-character subtypes (orientation)
The fourth character records setting and peer relationships (F91.0-F91.2) or the milder, younger-onset oppositional pattern (F91.3) — it is not a severity scale ranking how 'bad' the behaviour is.
- F91.0 — Conduct disorder confined to the family context
- Dissocial or aggressive behaviour that is confined almost entirely to the home and to interactions with immediate family members, with normal or near-normal peer relationships outside the home. (in_depth)
- F91.1 — Unsocialized conduct disorder
- The conduct-disorder pattern combined with significant, pervasive abnormalities in the child's relationships with other children — generally isolated, rejected, or unpopular with peers. (in_depth)
- F91.2 — Socialized conduct disorder
- The conduct-disorder pattern in a young person who is well integrated into a peer group (for example, a gang), with adequate peer relationships even though those relationships may themselves centre on antisocial activity. (in_depth)
- F91.3 — Oppositional defiant disorder
- A milder pattern, usually in younger children, of markedly defiant, disobedient, provocative behaviour without the more serious dissocial or aggressive acts that characterise F91.0-F91.2. (in_depth)
- F91.8 — Other conduct disorders
- A conduct-disorder pattern not fitting the other listed states. (taught for orientation only, not separately assessed in this pilot)
- F91.9 — Conduct disorder, unspecified
- Insufficient information to specify further. (taught for orientation only, not separately assessed in this pilot)
External causes to check first
Categories in this block require that the disturbance is not better explained by another external cause listed below. These short capsules are reminders, not full teaching modules.
Diagnostic principle capsule — Cross-situational pervasiveness
A shared reminder that F90 (hyperkinetic disorders) and F91 (conduct disorders) each turn, in part, on whether a behavioural pattern is confirmed across more than one setting or context, rather than confined to a single relationship, classroom, or household — though the two categories use this principle for different purposes: F90 requires cross-situational pervasiveness as a core, decisive diagnostic feature in its own right, while F91's fourth character (F91.0 confined to the family versus F91.1/F91.2 extending into peer and community life) uses setting information to describe the pattern's scope once a persistent conduct disturbance is already established.
What to check:
- Whether the behavioural pattern has been independently confirmed by more than one observer in more than one setting (for example, both a parent and a teacher, describing both home and school), rather than resting on a single informant's account of a single context.
- Whether a pattern confined to one unusually chaotic, understimulating, or high-conflict setting might instead reflect a situational response to that specific environment, rather than a pervasive, cross-situational pattern.
- For F91 specifically, that setting confirms scope (family-confined versus extending into peer and community life) only after a persistent conduct-disorder-level pattern is already established — it does not by itself establish that the pattern exists.
- That a single classroom's report, however detailed, is not sufficient on its own to confirm cross-situational pervasiveness — corroborating evidence from a second, independent setting should be sought before concluding the pattern is pervasive.
A full multi-informant clinical assessment gathering independent accounts from parents, teachers, and other caregivers across all the settings relevant to a child's daily life, which a single vignette can only approximate.
Common learner errors
Worked example — a pattern, or an isolated act?
A 12-year-old was caught shoplifting once with a group of friends. His parents and teachers report no other concerning behaviour before or since — no aggression, no destructiveness, no other rule violations — and he has otherwise age-typical peer relationships and school engagement.
- Step 1 — Check for a persistent pattern: only one incident is described, with no other dissocial or aggressive behaviour reported before or after.
- Step 2 — Check severity and context: shoplifting once, in a group, is a common adolescent experience and does not by itself indicate a pervasive pattern.
- Step 3 — Weigh what this means: F91 requires a persistent, repeated pattern; a single isolated act, however concerning to the family, does not meet this threshold.
- Conclusion — This does not support F91; an isolated incident without a broader pattern is not sufficient for a conduct disorder classification.
Interactive practice
- Type A A year-long pattern, not a phase Recognition · Foundational
- Type B One shoplifting incident Boundary discrimination · Intermediate
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.
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Explain why a single antisocial act is not sufficient for F91.
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.
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Explain the difference between unsocialized and socialized conduct disorder.
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.
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.
Rapid revision card
F91: A persistent, repeated pattern of dissocial, aggressive, or defiant conduct beyond ordinary childhood mischief — never a single act, and never a judgment on the child's character.
Decisive boundary cues:
- Repeated pattern over 6+ months -> weighs toward F91.
- One isolated incident, no other pattern -> weighs against F91.
- Confined to the family context only -> F91.0; poor peer relationships -> F91.1; well-integrated peer group -> F91.2.
Duration/course: A persistent pattern, typically over 6 months or more, is required — not a single incident.
Nearest alternatives: F90, F92, F93
Common mistake: Treating a single defiant or antisocial episode as sufficient evidence for a conduct disorder.
Retrieval prompt: What would you need to confirm before concluding this is a persistent pattern rather than an isolated incident?
(or use your browser's print function) Open full-page printable revision view →
Related glossary terms
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Conduct disorder
A repetitive, persistent pattern of dissocial, aggressive, or defiant behaviour exceeding ordinary childhood mischief or age-appropriate adolescent rebelliousness — a description of a behavioural pattern, never a judgment on a child's character or worth.
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Oppositional defiant behaviour
A milder pattern, usually seen in younger children, of markedly defiant, disobedient, and provocative behaviour without the more serious dissocial or aggressive acts that characterise other conduct disorders (F91.3).
Source basis
WHO ICD-10 international classification structure and code/title (ICD-10 Browser, 2019 English release).
Original educational synthesis, informed by general published clinical-education understanding of ICD-10 F90-F98; no verbatim CDDG or DCR text is reproduced. See Sources and Method for the full policy.
Last reviewed: 2026-09-17. Full policy: Sources and Method.
Educational resource only. This page and its cases teach ICD-10 classification reasoning using fictional scenarios. It does not diagnose real people and should not be used to assess yourself or anyone else. If you are concerned about a child's development, behaviour, or emotional wellbeing, please speak with a qualified professional for assessment and support.