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Fragile X Syndrome and Autism

Around 1 in 3 people with Fragile X Syndrome are also diagnosed with autism. Although not everyone with Fragile X Syndrome has autism, many of the behaviours associated with the condition are autistic-like.


Fragile X Syndrome is a genetic condition and is the most common inherited cause of learning disabilities, which can be identified with a blood test (read more here). Autism, on the other hand, is a description of a pattern of characteristics including difficulties with social interaction and communication, as well as restricted or repetitive interests or activities. Autism can be difficult to diagnose because there is no medical test so this is assessed by evaluating patterns of behaviour.


The exact causes of autism are still being researched, but a complex range of genetic and environmental factors are likely to be involved. The estimates of prevalence vary, but it is believed that approximately 1 in 100 people may have autism. You can read more about autism on the National Autistic Society website.



Many of the behaviours associated with Fragile X Syndrome are autistic-like, such as avoidance of eye contact, social withdrawal, communication difficulties and repetitive behaviours. Around one third of people with Fragile X have enough of these characteristics to meet the criteria to be diagnosed with autism. The proportion of people with Fragile X who have autism accounts for approximately 2-6% of all cases of autism, making Fragile X the most common, known single-gene cause of the condition.


It is important to note that not everyone with Fragile X Syndrome has autism; and though many will have at least some autistic-like behaviour, there are differences in what is driving the behaviour. In Fragile X, inattention, hyperactivity and anxiety interfere with learning from, or participating in, social interaction, resulting in autistic-like behaviours. Furthermore, autistic characteristics Fragile X become more prominent when there are more severe learning difficulties. In comparison, one of the core issues in autism is social interaction and the factors that lead to the autistic behaviours are believed to be more varied.


For more information on the shared characteristics between Fragile X and autism, and how they differ, download our handy guide explaining the similarities and differences.


Similarities and Differences between Fragile X Syndrome and Autism

Even when autism is present with Fragile X, the pattern of symptoms may be subtly different than in autism more generally. Namely, people with autism who have Fragile X tend to be more socially responsive (for instance, show more social smiling) but show more repetitive behaviours, when compared to people with autism who do not have Fragile X. Therefore, even when an individual is diagnosed with autism as well as Fragile X, it is important to understand their genetic condition. 

Similarities and differences (PDF)

Adapted from: Cornish, Turk & Levitas (2007)

The Importance of Understanding the Similarities and Differences


Autism is becoming ever more widely recognised and there is a fast-growing amount of evidence about different interventions to improve quality of life, which the Fragile X community may be able to learn from.


However, it is important that people who have Fragile X get the right diagnosis. Having the correct diagnosis allows for better understanding of the difficulties that the individual may face, and the best way to intervene. For instance, Fragile X is also associated with particular learning styles, and understanding these can be beneficial when teaching new skills (see our Education Booklet for further information). In addition, as Fragile X is an inherited condition, members of the immediate and wider family would benefit from genetic counselling if their relative is diagnosed. As a result, Professor Jeremy Turk (Consultant Child and Adolescent Psychiatrist & Fragile X Society Professional Advisor) recommends Fragile X testing for people showing autistic-like behaviour, particularly when they also have a learning disability.

Further Information on Fragile X and Autism

Get in touch with our Families and Professionals Advisors to get help and guidance on the link between Fragile X and autism.

This piece was written using information from the following sources:

Fragile X Syndrome and Autism: Common Developmental Pathways? by Kim Cornish, Jeremy Turk and Mark Levitas (2007)

The Fragile X syndrome–autism comorbidity: what do we really know? by Len Abbeduto, Andrea McDuffie and Angela Thurman (2014)

FXS and ASD: Similar But Different. National Fragile X Foundation

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