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Causes of autism spectrum disorder

By Raising Children Network
 
 

We don’t yet know exactly what causes autism spectrum disorder (ASD). In fact, it’s suspected that there may be several causes. Among these are brain development and genetic factors. Autism is not caused by anything that parents do or don’t do while raising their child.

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Brain development

In children with ASD, the brain develops differently from typically developing children:

  • The brain tends to grow too fast during early childhood, especially during the first three years of life.
  • The brain of an infant with ASD appears to have more cells than it needs, as well as inefficient connections between the cells.

Too many connections
It’s thought that the characteristic behaviours of autism come from difficulties with how the brain processes information (especially if the affected areas of the brain are those responsible for understanding emotions and language).

A young child’s brain is developing all the time. Every time a child does something or responds to something, connections in the brain are reinforced and become stronger. Over time, the connections that aren’t reinforced disappear – they are ‘pruned’ away as they’re not needed.

This ‘pruning’ is how the brain makes room for important connections – those needed for everyday actions and responses. It’s thought that, in children with ASD, this pruning doesn’t take place as much as it should – so information might be lost or sent through the wrong connections.

The lack of pruning might also explain why the brain seems to be growing faster than in typical development.

Genetics

Genetic factors seem to play a major part in ASD. For example, some families have multiple children diagnosed with ASD, and ASD also occurs in four times as many boys as girls. 

One specific gene is unlikely to be responsible for ASD. Rather, it might be that several genes combine and act together. 

It’s also possible that different gene combinations lead to different kinds of ASD, such as autistic disorder and Asperger’s disorder. 

ASD can also occur together with other genetic conditions, such as Fragile X syndrome.

One example of a gene identified in people with ASD is ‘neurexine 1’. This is a gene we all have, and it’s important for communication within the brain. Problems (or ‘disruptions’) in this gene are a known problem in ASD. But since the disruption by itself is not enough to cause ASD, this is an example of how multiple factors might be involved in causing ASD.

What about environmental factors?

To date, there is no solid evidence to show that ASD can be caused by anything in the environment, such as diet (either during pregnancy or once a child is born) or exposure to certain toxins. It is thought, however, that external factors might trigger ASD in a child who is already genetically prone to developing the condition.

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  • Last updated10-01-2009
  • Last reviewed03-11-2010
  • Acknowledgements

    Article developed in collaboration with Cheryl Dissanayake and Cherie Green, The Olga Tennison Autism Research Centre, La Trobe University.

References

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    Critchley, H. D., Daly, E. M., Bullmore, E. T., Williams, S. C. R., Van Amelsvoort, T., Robertson, D. M., et al. (2000). The functional neuroanatomy of social behaviour: Changes in cerebral blood flow when people with autistic disorder process facial expressions. Brain, 123(11), 2203-2212.

    Dawson, G. (2008). Early behavioral intervention, brain plasticity, and the prevention of autism spectrum disorder. Development and Psychopathology, 20, 775-803.

    Feng, J., Schroer, R., Yan, J., & al., e. (2006). High frequency of neurexin 1β signal peptide structural variants in patients with autism. Neuroscience Letters, 409, 10-13.

    Ingram, J. L., Stodgell, C. J., Hyman, S. L., Figlewicz, D. A., Weitkamp, L. R., & Rodier, P. M. (2000). Discovery of allelic variants of HOXA1 and HOXB1: Genetic susceptibility to autism spectrum disorders. Teratology, 62, 393-405.

    London, E., & Etzel, R. A. (2000). The environment as an etiologic factor in autism: a new direction for research. Environmental Health Perspectives, 108(S3), 401-404.

    Muhle, R., Trentacoste, S. V., & Rapin, I. (2004). The genetics of autism. Pediatrics, 113(5), e472-486.

    Persico, A. M., & Bourgeron, T. (2006). Searching for ways out of the autism maze: genetic, epigenetic and environmental clues. Trends in Neurosciences, 29(7), 349-358.