What causes Autism? What we suspect, know and don’t know about ASD

Once of the greatest and most persistent mysteries of autism is what causes the brain to develop so differently.

Seventy more years of methodological research point to the fact that the causes aren’t so obvious.  Recent research has coffees on biological factors that may lead to autistic behaviors, but there may be no causes of autism. Indeed, a variety of genetic factors are probably the ultimate cause of most cases. These may work independently, or in combination with environmental factors, to lead a child’s brain to develop so differently and result in autistic behaviors. 


To examine the influences of nature (genetics) and nurture (environment) on a given human quality, scientists studied twins.  

The first twin study of autism was conducted in 1977 on 11 identical and 10 fraternal twins across Great Britain, where at least one of the twins had autism. Concordance for identical twins was 36 percent, compared to 0 percent for the fraternal twins.

While the study was only small in size, it provided the first evidence that autism may be genetic in origin. Since this study, more than a dozen further twin studies have confirmed this original observation.

The best current estimate is that there is a 50-80 percent concordance for identical twins and a 5-20 percent concordance for fraternal twins. This indicates a strong genetic component to the condition. The figure for fraternal twins – 5-20 percent – also represents the chance of a couple who already have a child with autism having a second child with autism (referred to as the “recurrence risk”).

Once scientists have established that the cause of a disorder is influenced by genes, the next task is to identify the exact genes that might be involved. However, after several decades of intensive research, scientists could find no one genetic mutation that all individuals diagnosed with autism shared.

It was these findings (or lack of findings) that led scientists to stop thinking of autism as one condition with one cause. They started viewing it as many different conditions which all have relatively similar behavioural symptoms.

This new view of autism has proved extremely fruitful in discovering subtypes of autism. For example, a number of conditions have very clear genetic or chromosomal abnormalities that can lead to autistic behaviours.

These include disorders that have abnormalities of the chromosomes, such as Down syndrome. While no chromosomal condition itself accounts for any more than 1 percent of individuals with autism, when combined they account for approximately 10-15 percent of all individuals diagnosed with autism.

The exact genetic abnormalities that may lead to the remaining cases of autism are not completely clear. There are two reasons for this.

The first is that the genetic regions involved are likely to be very complex. Scientists have needed to develop new techniques to examine them.

The second is that it is probable the genetic mutations are very rare and complex. The DNA chain that forms our chromosomes contains more than 3 billion building blocks. To identify small pieces of DNA that may be linked to the development of autism among so many base pairs, scientists need to study a very large number of people with autism.

To date, no study has been able to examine the thousands of people necessary to identify with accuracy all of the small mutations that might lead to autism.

Environmental causes

Recognition that aspects of our environment may also contribute to autism. However, despite substantial research, no one environmental factor has yet been found to be a definite cause of autism.

The most widely used research technique to examine environmental risk factors for autism is epidemiology, which examines how often, and why, diseases occur in different groups of people.

Several environmental factors during prenatal life have been linked with autism.Bacterial or viral infections in the mother during pregnancy have been found to slightly increase the risk of autism in the offspring.

This could be due to the passage of harmful infectious organisms from the mother to the fetus through the placenta, or because the immune response of the mother may be detrimental to the developing brain of the fetus.

Several studies have found that fathers who are over 50 at the time of conception have a greater chance of passing on de novo mutations and also a greater risk of having a child with autism.

An obvious, but very important, observation is that not all people who are exposed to these factors are diagnosed with autism. One possible explanation for this is a phenomenon called gene-environment interaction, which is when the genetic make-up of two different people leads them to respond differently to an environmental factor.

Brain development

Scientists use a variety of techniques to understand the structure and function of the brain, such as magnetic fields, X-rays and radioactive chemicals. However, these tactics are not always able to  provide a full measure of the  complexity of how the brain operates.

It is also unlikely that autism affects just one area of the brain alone. The complex behaviours of individuals with autism, which include cognitive, language and sensory difficulties, make it difficult to pinpoint just one brain region that may be affected. Nevertheless, some promising leads have shown how different brain pathways may lead to autistic behaviours.

There is increasing evidence that differences in brain development may begin prenatally in some individuals with autism. Several studies of prenatal ultrasound measurements have found evidence for differences in the growth patterns of the brain in fetuses later diagnosed with autism. Newborns later diagnosed with autism are often also reported to have large heads at birth (‘macrocephaly’).

Other biological factors

There is preliminary evidence some but not all individuals with autism are exposed to higher levels of testosterone in the womb. Excessively high testosterone concentrations in the bloodstream can be harmful and cause cells to die, particularly within the brain, which is highly sensitive to changes in hormone levels.

One thought is that the pattern of cell death caused by high testosterone levels may alter brain development in a way that leads to autistic behaviours in childhood. This theory is still to be proven. Again, it is certain that not all individuals with autism are exposed to excessive levels of testosterone in the womb.

The link between gastrointestinal (gut) problems and autism is another scientific area that has received a great deal of attention. It is now well known that between 30 percent and 50 percent of individuals with autism experience significant gastrointestinal problems, such as diarrhoea, constipation, and an irritable bowel.

 Some scientists believe a disruption in the natural balance of these ‘good’ bacteria may be a potential cause of autism. Antibiotics, for example, are commonly used with infants in Western societies and are known to kill good bacteria along with the ‘bad’ bacteria for which they were prescribed.

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