26 Feb 2010

Research suggests that autism and intellectual disability have largely independent causes

research finds autism traits are strongly genetic

research finds autism traits are strongly genetic

New research by The Open University, King’s College London, the University of Cambridge and Birkbeck College suggests that the causes of autism and intellectual disability are mostly distinct.

The researchers found that behavioural and personality characteristics related to autism (also called ‘autistic traits’) are strongly genetic. Most importantly, they found that the genetic influences on autistic traits were largely distinct from the genetic influences on intellectual difficulties. This fits with the finding that although autism runs in families, relatives of children with autism do not tend to have intellectual disability.

“Autism and intellectual disability often occur together, and this has made many researchers think that the conditions must share the same genetic causes. Our research challenges this assumption,” says Dr Rosa Hoekstra, Lecturer in Psychology at The Open University, who led the study.

Autism can be diagnosed in people with all types of intellectual ability: some have profound learning difficulties, but others are extremely intelligent. The findings indicate that the characteristics of autism and intellectual disability are largely independent of each other. This contrasts with figures from clinics, which suggest that intellectual disability is common in people with an autism diagnosis. This may be because autism in very able children is going undetected.

”Diagnosis in able children is often late, after a long referral process,” says Dr Hoekstra. ”It is important that teachers and health workers are alert to the signs of autism in able children, so that problems can be detected early and appropriate services can be provided for the children and their families.”

It remains a puzzle why intellectual impairment is so common in people with autism, and whether this in part reflects a bias to diagnose autism in lower functioning individuals more readily than in intellectually able people.

Dr Hoekstra teamed up with two world-leading autism experts Professor Simon Baron-Cohen and Professor Francesca Happé from the University of Cambridge and King’s College London, respectively; and Dr Angelica Ronald from Birkbeck College – well-known for her study of twins and autism – to investigate the association between extreme autistic traits and intellectual disability, and to see if both traits share the same causes.

Rather than studying the association in children with diagnosed autism, in which the results may be biased, the scientists looked at the association between autistic traits and intelligence in a group of approximately 8,000 community-based twin pairs. Parents and teachers of the twins taking part in King’s College’s Twins Early Development Study were asked to rate the autistic traits of their children using a specially devised questionnaire, and the twins also completed intelligence tests. Hoekstra and her colleagues found that these two measures only showed a modest overlap, even in the children who showed many autistic traits or in the children who struggled most with the intelligence test.

Because the study focussed on twins, the researchers could also examine whether autistic traits and intelligence shared the same causes. Identical twins share all their genetic code, whilst non-identical twins, on average, share half of their genes, just like ordinary brothers and sisters. If identical twins resemble each other more closely than non-identical twins, this provides evidence for genetic influences. The researchers found that the strong genetic influences on autistic traits were largely distinct from the genetic influences on intellectual difficulties.

Notes to Editors
Full references to the relevant research papers:

Hoekstra, R.A., Happé, F., Baron-Cohen, S., Ronald, A., 2009. Association between extreme autistic traits and intellectual disability: insights from a general population twin study. British Journal of Psychiatry, 195, pp 531-536. See:

Hoekstra, R.A., Happé, F., Baron-Cohen, S., Ronald, A., In press. Limited genetic covariance between autistic traits and intelligence: findings from a longitudinal twin study. American Journal of Medical Genetics Part B: Neuropsychiatric Genetics. Published online, DOI: 10.1002/ajmg.b.31066.

For anyone wishing to find out more about autism:

The OU offers a science course Understanding The Autism Spectrum (SK124), which has already attracted many students, including parents of a child with autism as well as people on the autism spectrum themselves.

About research at The Open University:

- The Open University climbed 23 places to 43rd in the UK’s latest Research Assessment Exercise (RAE 2008), securing a place in the UK’s top 50 higher education institutions. Results showed that more than 50% of the University’s research is internationally excellent (3*), with a significant proportion world-leading (4*).

- The Open University is the UK’s largest university and the world leader in distance education, and is last year celebrated its 40th anniversary. It has more than 200,000 students in over 40 countries. Of these, more than 1,100 are postgraduate research students.

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