But genetics aren’t the whole story. “The general belief is… that there is a huge variety of different genes and very complex genetic architecture, and probably with the environmental influences as well, to trigger [autism]”, Sandin says.
To try to pinpoint the missing pieces, researchers have explored an array of behavioural and environmental risk factors. Some explanations have been debunked, such as the “refrigerator mother” theory that emotionally cold mothers contributed to autism in their children. Sandin is also sceptical of the idea that certain antidepressants used by parents are linked to autism risk in children, although there does appear to be an association with psychiatric disorders in parents.
There appears to be strong, though mixed, evidence that elevated autism likelihood is linked to higher parental age – possibly due to a higher likelihood of de novo (spontaneous) mutations in older fathers’ sperm. An increased risk of mutations may also help explain the correlation between preterm births and autism, as spontaneous mutations may contribute to the chances of preterm births.
Getty ImagesPreterm births are also related to social factors like low income and food insecurity, which may partly explain the steeper rise of autism diagnoses in minority children in the US.
In addition, there are suggestions that environmental exposures, including from pesticides and air pollution, may play a role.
Some studies also have found correlations with birth spacing and mothers’ health conditions, including diabetes, smoking and obesity. But these have proven difficult to untangle – it’s not clear whether maternal BMI directly contributes to autism, for example, or if it’s just that older mothers tend to have higher BMIs, Sandin says – and may be related to shared environmental conditions rather than a direct link.