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IQ and autism:

Updated: Jul 26, 2021

What do we measure with IQ tests?

Measuring non-autistic people by [the autistic] type of development would often find them failing miserably and appearing to be thoroughly ‘subnormal’ by ‘autistic’ standards. (Donna Williams)

Before we can discuss IQ tests and the way they are used with autistic population, we have to define what intelligence is.

What is intelligence?

And here we have a problem: there is no single definition of intelligence that has been universally accepted and we have to deal with a wide class of vague descriptions such as ‘the ability to carry on abstract thinking’, ‘an innate, general cognitive ability’ (Burt 1955), ‘the aggregate or global capacity of the individual to act purposefully, to think rationally, and to deal effectively with the environment’ (Wechsler 1958) and even ‘that quantity that the intelligence tests measure’.

Charles Spearman (1904) introduced the concept of general intelligence (“g” factor) that serves as the foundation for learning, and specific abilities (“s” factor). Spearman argued that “g” factor underlies all intellectual activities and functions and predict a person’s performance.

R. B. Cattell (1943) developed the concept of ‘general intelligence’ further by distinguishing two kinds of intelligence that made up the “g” factor, and showed how they differed in children and adults:

  • fluid intelligence (the ability to think and reason abstractly, to discriminate and to perceive relationships between things new or old, without prior practice or instruction): It is genetically inherited, which may account for individual differences. It increases until adolescence and then slowly declines, perhaps because of age-related changes in the brain;

  • crystallized intelligence (the store of knowledge gained from investing fluid intelligence in cultural activities, including skills, such as verbal comprehension and numerical facility, because these abilities rely on knowledge already acquired). It increases gradually over a lifetime and stays relatively stable until 65 years old, when it begins to decline.

According to Cattell, fluid and crystallized intelligence are fairly independent of each other, but having a higher fluid intelligence might lead to the broader and faster development of crystallized intelligence. Intelligence tests test at all ages the combined resultants of fluid and crystallized ability, but in childhood the first is predominant, whereas in adult life, owing to the recession of fluid ability, the peaks of performance are determined by the crystallized abilities (Cattell 1943).

Autism and Intellectual disability

ASDs are a heterogeneous group of neurodevelopmental conditions that vary in both aetiology and phenotypic expression. However, research has focused mostly on higher functioning autistic individuals, while non-verbal/ minimally verbal autism and what is called ‘autism with intellectual disability’ is understudied and these individuals are underrepresented. This is understandable, because for many reasons, it is difficult for them to participate in research projects.

Current estimates suggest the proportion of the population with ASD who have intellectual disability is 37% - 50% (Chakrabarti 2017; Russell et al. 2019; Stedman et al. 2018).

However, the term ‘autism with intellectual disability’ is somewhat misleading because intellectual disability in autism differs from that in non-autistic population. Unlike non-autistic children with intellectual disability who are delayed in both speaking and understanding language, and in all other areas of their development, children with ASD show specific delays in communication and socialization (Tantam 2013) and discrepancies in performance between verbal and non-verbal intelligence. While autistic children have uneven development (delays in some areas and not in others), non-autistic children with intellectual disability tend to be slower in all areas.

What do we measure with IQ tests?

As autistic people live in a different perceptual world from non-autistics, they develop different cognitive mechanisms and styles. So what do we really measure with the standard IQ tests that do not take into account all these differences? It is as if we tested the IQ of a blind person by asking him to name the colours of the objects in front of him. Even using his hands (tactile perception) he would not be able to pass the test successfully. Does it mean that he would be diagnosed as intellectually disabled?

Obviously, some autistic individuals may be intellectually disabled, just as some non-autistic people are. However, the poor results of the IQ tests may be accounted for by different reasons. As autistic individuals have different information-processing strategies and styles, they might struggle with tasks presented in a conventional, non-autistic way. For example, the autistic person either may not understand what is expected from her or may be unable to access his/her ‘mental database’ at the moment of testing. It can also be that the child working in ‘mono’ may be presented with multisensory information, or the child can understand much more than she is able to express with words or gestures. Besides, very often, smart autistic individuals are bored or even offended by the questions of the examiner and may give incorrect answers on purpose or refuse to cooperate altogether.

[Recently, some researchers have started developing (or adapting existing) tests to assess the cognitive potential of non-verbal or minimally verbal people with ‘severe autism’.[1] Some test different techniques (for instance, eye tracking or brain imaging) to reveal hidden abilities that standard IQ tests may overlook or underestimate.]

In addition to inadequate tests, the matter is complicated (and aggravated) by ‘unprofessional’ professionals: those who are inexperienced and lacking knowledge about autism but who are in the position to evaluate autistic individual’s abilities and deficits.

I remember being present at the evaluation of IQ of an autistic boy with behavioural problems by a clinical psychologist. The boy was offered tasks to measure his non-verbal intelligence. The performance was poor because the boy’s attention was anywhere but at the task (was he bored?) The mother could not believe her eyes, as he was able to do similar tasks at a very early age. The verdict was: the boy’s IQ was about 40. Some additional information: the boy was very good at mathematics and language. He could read and write. At school he actively participated in all activities with minimal support. So what was this evaluation about?


[1] There is no agreed definition for ‘severe autism’/ ‘low-functioning autism’, but traditionally, it involves little or no speech, a low IQ (20 to 69), inability or difficulty to master life skills, to perform everyday tasks and live/ function independently.

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