Autism spectrum disorder and intellectual disability

Some autistic children have additional developmental, psychiatric, or medical diagnoses or conditions (eg epilepsy).

These are sometimes described as co-morbidities or co-morbid disorders. In other words, co-morbidities are additional conditions that impact on somebody at the same time as a diagnosis such as autism. Some autistic people can have extremely high IQ and some autistic people can have average IQ. However, intellectual disability (ID) is common in autism, with estimates of the overlap varying between 38%-67% (with an average of 57%-58%) of autistic children having ID.

About 1% of the general population is thought to have an ID, and about 10% of individuals with ID have autism, or autistic traits. However, a much higher percentage of autistic people have ID.

The most recent study conducted by the Centers for Disease Control (CDC) studied records from 2008, finding that:

• 38% of autistic children had ID. 24% of autistic children were considered in the borderline range in terms of intellectual ability (an IQ of 71–85); 38% had IQ scores over 85, considered average or above average.
• A higher proportion of autistic females had ID compared with males: 46% of autistic females had ID, compared with 37% of males (note, autism is almost five times more prevalent in males than females.)

Since the CDC has been measuring prevalence rates of autism and co-occurring ID, the rate of autistic people who do not have co-occurring ID has been rising faster than the rate of autistic people and ID. Additionally, in the CDC’s most recent report, autism prevalence was typically higher in states which had a greater percentage of children with IQs above 70.

Measures of IQ less than 70 are used to indicate the intellectual functioning aspect of ID. This is because an IQ of 70 is significantly below the average population IQ (100). It is possible to have a below average IQ (e.g. 80), and not get a diagnosis of ID.

Scientists are trying to determine if there is a genetic link between ID and autism. Certain genetic syndromes (Fragile X, Rett, Tuberous Sclerosis, Down’s Syndrome, phenylketonuria, CHARGE, and Angelman) are associated with ID and also have a higher incidence of autism. However, other research has shown that ID is associated with a high number of deletions within an individual’s genetic code, whereas autism is associated instead with a high number of duplications. Regardless of the possibility of a causal connection, it is recognised that autistic people and those with ID share common struggles, particularly with respect to social and communication skills, which are necessary components of the autism diagnosis.

The topic on ID highlighted that social abilities, such as empathy, are an important aspect of adaptive functioning that people with ID may have difficulty with. The topic on autism also discussed apparent difficulties with empathy. Therefore, when a health professional evaluates a child, difficulties with empathy or making friends may be attributable to autism and/or ID. There is a modest link between autism and ID, but autism and ID are thought to be substantially genetically independent of each other. On the autistic spectrum, there are autistic people, people with ID, and people with both.