The effects of intellectual functioning on the range of core symptoms of autism spectrum disorders.
IQ shapes how autism traits look in adults with ID, so factor it into every diagnostic and planning meeting.
01Research in Context
What this study did
Matson et al. (2008) compared adults who have both autism and intellectual disability with adults who have only intellectual disability.
They measured how many core autism traits each group showed and checked whether IQ level changed the picture.
The design was quasi-experimental: the researchers sorted people by existing diagnoses instead of randomly assigning them.
What they found
Adults with ASD plus ID displayed more classic autism symptoms than adults with ID alone.
IQ mattered across the whole sample, but once you looked only at the autism group, IQ no longer predicted symptom level.
In plain words, knowing a person’s IQ helps you judge symptom severity only when you compare different diagnostic mixes.
How this fits with other research
Matson et al. (2009) extended the same adult sample and showed that adding ASD to ID lowers Vineland adaptive scores even further.
Busch et al. (2010) also extended the work: when epilepsy joins ASD and ID, social skills drop again.
Ohan et al. (2015) conceptually replicated the IQ-moderation idea in youths, but found a twist—higher autism severity sometimes linked to better adaptive skills depending on age and IQ.
This creates an apparent contradiction: in adults, higher IQ softens symptom impact across groups, while in kids it can boost adaptive growth even when symptoms look worse. The difference is outcome type (symptom count versus daily-life skills) and age.
Why it matters
Always record IQ when you assess adults with ID. A low IQ can inflate autism checklist scores, so adjust your clinical cutoff or you may over-diagnose. If the person already has an ASD diagnosis, IQ won’t fine-tune your picture of core symptoms, but it will still guide goal setting and funding levels.
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02At a glance
03Original abstract
Autism spectrum disorders (ASD) are a class of conditions categorized by communication problems, ritualistic behaviors, and inappropriate social behaviors. While there is much evidence to support a genetic link for ASD, an identified genetic marker remains elusive. As such, practitioners place considerable emphasis on traditional measures of intelligence and adaptive behavior to aid in diagnosis. Despite the fact that these measures are commonplace, little research has been conducted to shed light on whether deficits in intellectual functioning affect the range of core symptoms for ASD. This study represents a first attempt to determine whether level of IQ has an effect on the expression of ASD symptoms in adults with intellectual disability (ID). Three hundred and six adults, 151 with both ASD and ID and 155 with ID alone, were evaluated with respect to the nature and extent of their ASD symptoms and intellectual functioning. Individuals with autism displayed a higher number of symptoms than those with pervasive developmental disorder-not otherwise specified (PDD-NOS) on all three domains of impairment (social, communication, repetitive behaviors). As expected, persons with ID alone evinced far fewer symptoms than both these groups. IQ level was found to be a moderator for expression of ASD symptoms for the entire sample but not for the autism group.
Research in developmental disabilities, 2008 · doi:10.1016/j.ridd.2007.06.006