Prevalence of Autism Spectrum Disorder in Individuals with Borderline Low IQ
Approximately 30% of individuals with Autism Spectrum Disorder (ASD) have co-occurring intellectual disability, with the cognitive profile in ASD ranging from severely impaired to superior ability. 1 While the evidence does not provide specific prevalence data for the narrow "borderline low IQ" range, the relationship between ASD and cognitive functioning follows a clear pattern that can guide clinical understanding.
Cognitive Distribution in ASD
The cognitive profile in ASD populations breaks down as follows:
- 50% have severe to profound intellectual disability (IQ typically <35-40) 1
- 35% have mild to moderate intellectual disability (IQ typically 40-70) 1
- 15-20% have IQ in the normal range (IQ ≥70) 1
Borderline intellectual functioning (IQ 70-85) would fall within or adjacent to the mild intellectual disability and normal range categories, representing a substantial portion of the ASD population. 1
Key Clinical Considerations
Cognitive level is the primary driver of behavioral presentation variability in ASD, more so than the core social communication deficits themselves. 1 This means individuals with borderline low IQ and ASD will show:
- More pronounced functional impairment than those with average or above-average IQ 1
- Greater challenges with adaptive functioning despite having cognitive abilities in the borderline range 1
- Higher rates of co-occurring conditions, as approximately 90% of individuals with autism have at least one additional medical or mental health condition 1
Specific Neurobehavioral Patterns
In cases where ASD co-occurs with reduced cognitive functioning (including borderline range):
- Verbal skills are typically more impaired than nonverbal skills in classic autism presentations 1
- Working memory and processing speed deficits are prominent and partially mediate IQ reductions 1
- Receptive and expressive language test scores show decreased performance 1
- Motor dysfunction is observed in early developmental course 1
Diagnostic Implications
The presence of borderline low IQ does not preclude ASD diagnosis but complicates the clinical picture. 1 Clinicians must avoid diagnostic overshadowing—the tendency to attribute all symptoms to intellectual limitations while missing the distinct social communication and restricted/repetitive behavior patterns that define ASD. 1
Comprehensive neurobehavioral evaluation should include:
- Efficient indicators of global ability with both verbal and nonverbal components 1
- Measures of sustained attention, working memory, and processing speed 1
- Assessment of receptive, expressive, and pragmatic language skills 1
- Evaluation of fine and gross motor skills 1
- Adaptive function assessment 1
- Autism-specific measures including parent report and clinician observation 1
Common Pitfalls
Do not assume that borderline cognitive functioning explains social deficits. The social communication impairments in ASD are qualitatively different from those seen in isolated intellectual disability—they involve specific deficits in joint attention, social reciprocity, and nonverbal communication that persist even when accounting for cognitive level. 1
Recognize that individuals with ASD and borderline IQ face compounded functional challenges, with lifetime costs estimated at $2.4 million for ASD with intellectual disability compared to $1.4 million without. 1 Early identification and intervention are critical, as behavioral interventions show small to medium effect sizes for improving language, play, and social communication. 2