What causes autism? What we know, don’t know and suspect
The best current estimate is that there is a 50-80% concordance for identical twins and a 5-20% concordance for fraternal twins. This indicates a strong genetic component to the condition. The figure for fraternal twins – 5-20% – 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% of individuals with autism, when combined they account for approximately 10-15% 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.