How to Differentiate ADHD from ASD
The key to differentiating ADHD from ASD lies in identifying core social-communication deficits and the quality of repetitive behaviors: children with ASD demonstrate fundamental impairments in joint attention, nonverbal communication, and social reciprocity that are absent in pure ADHD, while their repetitive behaviors serve self-regulatory functions rather than being driven by impulsivity. 1
Core Distinguishing Features
Social Communication Patterns
Children with ASD show qualitatively different social deficits compared to ADHD:
Joint attention deficits are pathognomonic for ASD—look specifically for failure to point to share interest (not just to request), lack of showing objects to others, and absence of gaze-checking to share experiences. 2, 1 These behaviors consistently differentiate autistic children from those with ADHD at 20 and 42 months. 2
Failure to respond to name at 12 months is highly specific (86%) for ASD and distinguishes it from ADHD and other developmental delays. 1
Eye contact quality differs between disorders: ASD involves qualitatively impaired, poorly modulated eye contact that lacks social intent, whereas ADHD may show reduced eye contact simply due to distractibility. 1 Children with ASD use eye contact less frequently and inappropriately to regulate social interaction. 2
Conventional gesture use is markedly impaired in ASD (waving, nodding, shaking head) but preserved in ADHD. 2
Use of others' bodies as tools (e.g., moving parent's hand to open door) is characteristic of ASD but not ADHD. 2
Repetitive Behaviors: Quality Over Quantity
The function and nature of repetitive behaviors differ fundamentally:
ASD repetitive behaviors include hand flapping, finger flicking, rocking, spinning, and atypical movements that serve self-regulatory functions, are ego-syntonic, and increase with stress or excitement. 1 These are qualitatively different from simple hyperactivity. 2
ADHD repetitive behaviors consist of fidgeting, difficulty remaining seated, and excessive talking driven by impulsivity and hyperactivity rather than self-regulation. 1
Emotional and Behavioral Regulation
Children with ASD at 24 months demonstrate lower positive affect, higher negative affect, and poor effortful emotion regulation that is qualitatively different from ADHD impulsivity. 1
Sensitivity to social reward is markedly reduced in ASD, with difficulty controlling behavior in response to social cues, whereas children with ADHD typically respond to social reinforcement. 1
Diagnostic Evaluation Algorithm
Step 1: Establish Developmental Timeline
ADHD requires symptom onset before age 12, presence across multiple settings since early childhood, and persistence regardless of sleep quality or other factors. 2, 1
ASD symptoms typically manifest in early development, though some children show regression after a period of apparently normal development. 2
Step 2: Multi-Informant Assessment
Obtain information from at least two sources (parents, teachers, coaches, school counselors) documenting symptoms in more than one major setting. 2
Use DSM-5-based rating scales with age- and gender-specific norms for ADHD. 2
For ASD, use standardized tools: ADOS-2 for direct observation of social-communication behaviors and ADI-R for developmental history. 1 The M-CHAT is recommended for screening at 24 months. 1
Step 3: Assess Specific Discriminating Features
At 24 months, focus on:
- Directing attention (showing objects)
- Attention to voice
- Pointing for interest (not just requesting)
- Use of conventional gestures 2
At 36 months, assess:
- Use of other's body as tool
- Attention to voice
- Pointing behaviors
- Finger mannerisms 2
From 38-61 months, evaluate:
- Impaired nonverbal behaviors (eye contact, facial expressions, body postures) used to regulate social interaction 2
Step 4: Differentiate from Language Disorders
Two behaviors consistently differentiate ASD from language impairment: pointing for interest and use of conventional gestures. 2
Children with language disorders impact socialization but maintain social intent and reciprocity that is absent in ASD. 2
Critical Comorbidity Consideration
Screen for both conditions simultaneously, as they frequently co-occur:
23.3% of adults with ADHD meet ASD diagnostic classification on ADOS-2, with social reciprocal interaction scores tending to be higher while restricted/repetitive behavior scores remain low. 3
Screen for ADHD symptoms in every child with ASD, as inattention, impulsivity, and hyperactivity are among the most frequent associated symptoms. 1
Screen for ASD features in children with ADHD who have prominent social difficulties beyond what would be expected from impulsivity alone. 1
Mandatory Additional Screening
Both conditions require screening for:
- Anxiety and depression (increased risk in both, especially adolescents) 2, 1
- Learning disabilities and language disorders 2, 1
- Sleep disorders 2, 1
- Oppositional defiant disorder and conduct disorders 2, 1
- Tic disorders 2, 1
Common Diagnostic Pitfalls
Avoid diagnostic overshadowing: The tendency to fail to diagnose comorbid conditions when a more noticeable condition is present. 2 This is particularly problematic when ASD is obvious and coexisting ADHD is missed, or vice versa.
Do not rely solely on hyperactivity: Hyperactivity can occur in both disorders but serves different functions—impulsivity-driven in ADHD versus self-regulatory in ASD. 1
Recognize that anxiety symptoms overlap: Excessive worry, need for reassurance, inability to relax, and self-consciousness occur in both ASD (particularly higher-functioning individuals) and anxiety disorders, but ASD is distinguished by prominent social-communication impairments and lack of developed social insight. 2
Adolescents present unique challenges: They are less likely to exhibit overt hyperactive behavior, tend to minimize their own problematic behaviors, and must have documented manifestations before age 12 to meet ADHD criteria. 2 Consider whether mimicking conditions (substance use, depression, anxiety) are present. 2