Heritability of Autism Spectrum Disorder: With and Without Intellectual Deficits

In accordance with the belief that inclusion centers and autism awareness campaigns should be up to date with each dimension of Autism Spectrum Disorder most of all, we at Small Steps Big Dreams in Dubai, UAE keep a keen eye on new studies relevant to the condition whenever they are published. However, our work does not end there. We must share the knowledge, so as to remove social stigma and stereotypes against autistic individuals, and generate an environment of empathy and compassion towards them. In hopes of achieving this atmosphere of understanding, this article will shed light on one of the many facets of the condition. In particular, this article highlights how cognitive ability can introduce variation in the heritability of Autism Spectrum Disorder.


What are Intellectual Disabilities?

Intellectual disabilities were previously listed in the earlier editions of the Diagnostic and Statistical Manual of Mental Disorders (DSM) as “mental retardation”. However, the fifth edition (DSM-5) changed the diagnostic label formally to “intellectual disabilities” (or “intellectual development disorder”), marking a shift in attitude towards such impairments. Not only of social response to it, but (perhaps by extension) of medical response as well. Such can be seen in the way these disabilities were diagnosed. Prior to the change, intellectual impairments were diagnosed merely through intelligence quotient tests (or IQ tests, for short). Afterward, the testing became more complex to include clinical assessment along with standardized testing. Intellectual disabilities involve deficits in two major areas:

  • Intellectual functioning, for example learning, judgment and problem solving
  • Adaptive functioning, for example day to day activities, like communication


Autism Spectrum Disorder and Intellectual Disabilities

There is a range of signs and symptoms that are associated with Autism Spectrum Disorder. Among them, the most noted is perhaps the intellectual difficulties that one is posed with because of the condition, despite the fact that only about a third of those with ASD have intellectual impairments. According to the DSM-5, intellectual development disorder and Autism Spectrum Disorder can manifest in a comorbid state. It includes the intellectual problems autistic individuals have to deal with, which comprise of language delay as the most defining feature.

In fact, ASD and intellectual disabilities were often misconstrued as similar conditions, and even medical professionals could not identify enough variation between the two because of the lack of biological distinction between the two and genetic overlapping. While science has made significant strides in de-blurring the similarities, there is still a long way to go before we can be sure where these two conditions intersect and why.


Autism Spectrum Disorder Influenced by Family History

It is believed that autistics with intellectual disabilities have an IQ as low as 70. According to recent research, the genetic structure differs for those with Autism Spectrum Disorder and no intellectual deficits, and those with Autism Spectrum Disorder with intellectual deficits; as studies show that autistic individuals with lower intelligence quotients are more likely to have spontaneous- or de novo- gene mutations, as compared to autistics with higher IQs. Building on this theory, a study lead by associate professor of epidemiology and biostatistics at Drexel University, Brian Lee focuses on how Autism Spectrum Disorder with intellectual impairments is less heritable than the condition with these deficits.

Starting the study, Lee and his colleagues turned to the Stockholm Youth Cohort to analyze diagnostic records from 567,436 people born in Sweden from the years 1984 to 2009. They also made use of the national registries to find out whether any of the parents, siblings, aunts, uncles and/or cousins of the participants were diagnosed with ASD. In consequence, they found that individuals with an autistic relative stand an increased chance of having Autism Spectrum Disorder, with or without intellectual impairments. In fact, the more closely related the autistic relative is, the higher the familial risk of developing ASD. Moreover, they found that the strongest risk association is with autistic mothers.

Here, it may be significant to note that the research did not include any data on whether or not the relatives have intellectual impairments. Lee claims that one of the reasons this was done was because the two conditions could have different diagnosis patterns over time, which would have made the results questionable and thus more or less useless.


Variation in Familial Risk of Autism Spectrum Disorder by Cognitive (Dis)Ability

The research, through the analytical data provided, corroborates the idea that familial risk does play a part in developing Autism Spectrum Disorder. However, it does that theory one better by differentiating between ASD with and without intellectual disabilities.

They use ASD+ID to describe Autism Spectrum Disorder with intellectual disabilities, and ASD-ID to denote Autism Spectrum Disorder without intellectual disbailities.

Lee et al found that the relatives of individuals with Autism Spectrum Disorder that is without intellectual impairments are more likely to be autistic, as compared to those with Autism Spectrum Disorder with intellectual disabilities. Furthermore, the closer the relation, the higher the odds are of having the condition. For example, one who has an autistic mother is about 20 times as likely to have ASD-ID, as one without, and 11 times as likely to have both ASD and ID.

Dorret Boomsma, professor of biological psychology at Vrije University, notes that the research also highlights that more severe intellectual impairments tend to be less heritable than intellectual disabilities that are less severe in individuals who do not have ASD.

Therefore, Lee et al have shown that heritability and genetics may play a more significant part in the development of ASD than had initially been realized. Boomsma recommends future studies to factor in whether or not autistic parents have intellectual disabilities, and how big families are with children who are born with intellectual disabilities.


The Heritability of Autism Spectrum Disorder: With Intellectual Deficits and Without

It is becoming excessively clear that inheritance is more significant to Autism Spectrum Disorder research than was earlier assumed. The study conducted by Lee et al clarifies that ASD+ID is less heritable than ASD-ID. This research stands as a good foundation for further research into the genetic contribution of Autism Spectrum Disorder, and can prove to be a game changer for ASD diagnoses if better understanding is achieved.

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