Possible Causes of Autism
To date, there is no accepted single cause of Autism, though there are numerous theories.
It is becoming apparent that:
- Autism is most probably caused by multiple factors interacting in complex ways (i.e. genes, environment and brain development).
- Autism is not etiologically homogeneous. That is, there are probably numerous sub-types of Autism each with differing etiologies. For example, there is evidence of a sub-group of children diagnosed with ASD (20-30%) who show skill regression between 18 – 24 months after apparently typical initial development (Lainhart et al, 2002) while other children diagnosed with ASD show consistently delayed development from the beginning.
Genetics have been shown to play a role but do not explain the full picture or the recent increase in diagnoses. Studies have shown that if one identical twin has the diagnosis, then there is a 60% chance that the other twin will develop Autism. This concordance is hardly ever seen with non-identical twins (Muhle et al, 2004). When a wider definition of the Autism spectrum is used, the probability rates jump to 92% for identical twins and 10% for non-identical twins (Muhle et al, 2004). The probability of receiving an ASD diagnosis when another sibling has already been diagnosed is estimated between 2 and 14%, a 10 to 20-fold increase over the general population incidence (Hertz-Picciotto et al, 2006). Extensive genetic studies have revealed hundreds of genes linked to autism. (Rylaarsdam and Guemez-Gamboa, 2019). ASD is now understood to be a disease of complex interaction between genetics and the environment, with heritability estimates ranging from 40 to 80% (Chaste and Leboyer, 2012).
Environmental Risk Factors For Autism
A 2017 article in The National Center for Biotechnology Information reported that the incidence of ASD diagnosis in the United States increased from 4-5 per 10,000 children in the 1980s to 30-60 per 10,000 in the 1990s, a ten-fold increase in as many years. According to the CDC’s Autism and Developmental Disabilities Monitoring (ADDM) Network, the prevalence of Autism in the United States increased from 1 in 150 children in 2000 to 1 in 54 children (1 in 37 boys, 1 in 151 girls) today. Although a portion of this increase is undoubtedly due to greater clinician awareness of the disorder and wider inclusion criteria, this cannot explain such a rapid and dramatic increase. More and more researchers are turning to environmental causes that contribute to autism as a central hypothesis.
Progress has been made toward understanding different environmental risk factors, and the clearest evidence involves events before and during birth, such as: advanced parental age at time of conception, prenatal exposure to air pollution, metals or certain pesticides, maternal nutrition, obesity, diabetes, or immune system disorders, extreme prematurity or very low birth weight, and any birth difficulty leading to periods of oxygen deprivation to the baby’s brain. However, these factors alone are unlikely to cause autism. Rather, they appear to increase a child’s risk for developing autism when combined with genetic factors. (National Institute of Environmental Health Sciences, 2021). There is also growing interest in the role of the immune system in Autism etiology, with the suggestion that certain chemicals (widely present in our environment) may be responsible for observed inflammation in the brains of some of those on the Autism spectrum.
It is widely accepted that atypical brain development underlies the development of the observable symptoms of Autism (see Autism Symptoms & Characteristics). How genes and the environment interact to cause the brain to begin developing differently is still being debated. What is clear is that these differences in brain development can be traced to either before birth or very soon after birth even though the behavioral and social signs of Autism tend not be observable until after 18 months of life. The exact nature of the brain differences is not clear either.
Studies have shown differential development in many brain areas including the frontal and temporal lobes, the cerebellum, and the sub-cortical amygdala and hippocampus (ex. Barnea-Goraly et al, 2014, Nordahl et al, 2020, Stoodley, 2014). Other studies point to the patterns of connectivity between and within brain areas as the issue (e.g. Courchesne and Pierce, 2005) rather than specific loci, while other researchers focus on specific types of neurons (e.g. mirror neurons) or patterns of neuronal activation (e.g. Bernier et al, 2007).
A 2014 study by neuroscientists at Columbia University Medical Center suggests that children affected by autism have a surplus of synapses, or connections between brain cells. Some children who are later diagnosed with autism also have excess cerebrospinal fluid — the liquid that surrounds the brain — compared with their non-autistic peers (Shen et al, 2018). A solid body of evidence suggests that white matter, the bundles of long neuron fibers that connect brain regions, is also altered in people with autism (Frazier et al, 2012, Thompson et al, 2020). However, scarcity of evidence, methodological challenges and conflicting findings have not yet allowed precise conclusions to be drawn about either the specific brain regions affected or the mechanism of development that lead to observed brain differences.
Due to the lack of consensus about what causes Autism, there is not yet any widely accepted medical treatment for Autism (although growing evidence for some bio-medical approaches, see Autism Treatment Options). However, there are other ways to help children and adults with Autism that are effective and have seen great successes – see Autism Treatment Options.