The underlying causes of autism are still not well understood, and are likely multifactorial and complex. Autism is fundamentally a disruption of the brain's neurological processes. This can occur within the neurons themselves, at the neural synapses (connections between neurons), or at a more structural/neural organizational level. As has been pointed out elsewhere, the diversity of autism is very great and this is certainly recapitulated at even the most basic neurobiological levels. Despite the high prevalence of autism, the complexity and diversity of the disorder has made it hard to study autism in labs, in epidemiological settings, and in treatment settings.
Nonetheless, it's possible to break out the causes of autism into two main groups:
1. Genetics. Based on large twin studies and family studies, it is believe that the underlying between 50 and 80% of autism cases are due to a genetic mutation, set of genetic mutations or otherwise influenced by genetics. The advance of sequencing technology in the past five years has made possible to sequence the genes in the human genome across thousands individuals with autism and their families. This has given rise to the idea that "de novo" mutations, new mutations which arise in the child, are a primary cause of autism. Every newborn has a set of new mutations, which are mostly benign or inconsequential. However, a small fraction of new mutations are detrimental to the production of key molecules or proteins needed by neurons to function properly. Researchers have identified that perhaps 20-30% of cases of autism have a new mutation which could plausibly underly autism. However, these mutations are incredible diverse-- the same de novo mutation is seen more than once only very rarely, and the top 5-10 most commonly mutated genes account for only ~1% of autism. The long tail makes it necessary to sequence tens of thousands of individuals to pick up signal. A second set.
2. Environment. It's thought (but poorly understood) that certain environmental factors, especially during pregnancy, could also underly autism. Alcohol, for example, can cause Fetal Alcohol Syndrome which can present with autism-like features. Unfortunately, this half of the equation is not my subject expertise so I'll leave it at that.
As a note about searching for information: While google can be unreliable, Google Scholar (https://scholar.google.com/) can be quite helpful.
Nonetheless, it's possible to break out the causes of autism into two main groups:
1. Genetics. Based on large twin studies and family studies, it is believe that the underlying between 50 and 80% of autism cases are due to a genetic mutation, set of genetic mutations or otherwise influenced by genetics. The advance of sequencing technology in the past five years has made possible to sequence the genes in the human genome across thousands individuals with autism and their families. This has given rise to the idea that "de novo" mutations, new mutations which arise in the child, are a primary cause of autism. Every newborn has a set of new mutations, which are mostly benign or inconsequential. However, a small fraction of new mutations are detrimental to the production of key molecules or proteins needed by neurons to function properly. Researchers have identified that perhaps 20-30% of cases of autism have a new mutation which could plausibly underly autism. However, these mutations are incredible diverse-- the same de novo mutation is seen more than once only very rarely, and the top 5-10 most commonly mutated genes account for only ~1% of autism. The long tail makes it necessary to sequence tens of thousands of individuals to pick up signal. A second set.
2. Environment. It's thought (but poorly understood) that certain environmental factors, especially during pregnancy, could also underly autism. Alcohol, for example, can cause Fetal Alcohol Syndrome which can present with autism-like features. Unfortunately, this half of the equation is not my subject expertise so I'll leave it at that.
As a note about searching for information: While google can be unreliable, Google Scholar (https://scholar.google.com/) can be quite helpful.
Some additional resources:
[1] A review on the de novo genetic hypothesis for ASD that I wrote a few years back: http://www.ncbi.nlm.nih.gov/pubmed/24387789
[2] A large sibling/twin study from California about that heritability of autism: http://www.ncbi.nlm.nih.gov/pubmed/21727249
[3] A recent large-scale sequencing study for autism (sorry, paywall): http://www.ncbi.nlm.nih.gov/pubmed/25363768
[4] A review of ASD epidemiology and etiologies: http://www.ncbi.nlm.nih.gov/pubmed/17367287
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