Aberrant Behavior Checklist

The ABC (Aberrant Behavior Checklist) is a 58-item caregiver-rated checklist measuring irritability, social withdrawal, stereotypy, hyperactivity and inappropriate speech in people with intellectual disability or autism. It is widely used as an outcome measure in clinical trials ¹.

What does the ABC measure?

The Aberrant Behavior Checklist (ABC) is a validated 58-item caregiver-rated scale developed to assess problem behaviours and treatment effects in individuals with intellectual disabilities ¹. Originally validated in residential settings for adults with ID, the ABC has since been adapted as the ABC-Community (ABC-C) for outpatient and home settings, making it one of the most widely used behavioural outcome measures in autism spectrum disorder (ASD) clinical trials.

It measures five behavioural domains: irritability, lethargy and social withdrawal, stereotypic behaviour, hyperactivity and noncompliance, and inappropriate speech. It does not assess social communication or restricted and repetitive behaviours, so it measures the severity of associated behavioural symptoms rather than the presence of a diagnosis.

The ABC Irritability subscale (ABC-I) has served as the primary efficacy endpoint in the pivotal registration trials for both risperidone and aripiprazole, both FDA-approved for the treatment of irritability associated with autism spectrum disorder in children and adolescents.

Placebo-controlled trials that measured efficacy with the ABC-I include:

• Aripiprazole: a fixed-dose trial ³ and a flexible-dose trial ⁶.
• Risperidone: the RUPP Autism Network trial ⁴, a trial in children aged 5 to 12 ⁷ and a dosing trial ⁸.

How do you give the ABC?

The ABC is caregiver-rated, not clinician-administered. A parent, teacher, or other regular caregiver who knows the individual well completes the 58-item checklist based on observations over the past week.

Each of the 58 items is rated on a 4-point scale (0 = not at all a problem, 1 = the behaviour is a problem but slight in degree, 2 = the problem is moderately severe, 3 = the problem is severe in degree). Items are summed within each subscale, scores are not averaged.

How do you score and interpret the ABC?

Because the ABC produces five independent subscale scores rather than a single total, there are no validated overall severity cutoffs — a score interpreter is not applicable for this scale. Each subscale must be interpreted on its own, in the context of an individual's history and clinical presentation.

The five-factor structure came from separate factor analyses of two samples (418 and 509 residents) with moderate-to-profound intellectual disability ¹. Subscale scores are raw sums. The table below lists each subscale with its item count and maximum possible score.

Common clinical trial thresholds

Across pharmacotherapy trials using the ABC as an outcome measure, two conventions have become standard, though they are not derived from a single defining paper:

Trial entry (eligibility). Pharmacotherapy trials for irritability in autism spectrum disorder have commonly required a minimum ABC-I subscale score at baseline as an eligibility criterion — the exact value varies by trial but is typically in the range of ≥18. This is a commonly used convention representing clinically significant behavioural impairment, not a threshold derived from a single defining psychometric study.

Responder definition. The landmark RUPP Autism Network risperidone trial ⁴ defined a positive response as two things together:

• at least a 25% decrease in the ABC-I Irritability score ⁴, and
• a rating of "much improved" or "very much improved" on the Clinical Global Impressions-Improvement (CGI-I) scale.

This dual criterion, symptom reduction plus global clinical impression, has since been adopted as a convention in later ASD pharmacotherapy trials.

Subscale II (Lethargy/Social Withdrawal). In trials involving individuals with intellectual disability and in some ASD studies, ABC-II is tracked as a secondary endpoint to monitor sedation-related withdrawal as a potential adverse effect of antipsychotic treatment, independent of its role as a symptom subscale.

Psychometric properties

In the original psychometric study ², internal consistency and test-retest reliability of the checklist were very good. Interrater reliability varied across raters and subscales, ranging from mediocre to good, and was generally in the moderate range and acceptable for research purposes. Validity was supported for most subscales by comparing scores across residents with differing degrees of social adaptation, against adaptive behaviour scales, and against direct behavioural observation.

Limitations

• Not diagnostic. The ABC rates the severity of associated behaviours; it cannot establish or rule out autism or intellectual disability.
• No total score or severity bands. Subscales are interpreted individually, and the trial thresholds above are conventions rather than validated cutoffs.
• Rater dependence. Scores rely on a caregiver's observations over the past week, and interrater agreement in the original study was only moderate.
• Original validation sample. The factor structure was derived in residents with moderate-to-profound intellectual disability; community use relies on the later ABC-Community adaptation.