brainscanEnvironmental factors are more important than previously thought in understanding the causes of autism, and equally as important as genes, according a study of how autism runs in families.

The study, which was led by researchers at King’s College London, Karolinska Institutet in Sweden and Mount Sinai in the US, also found that children with a brother or sister with autism are 10 times more likely to develop autism. 

In addition, children are 3 times more likely to develop the condition if they have a half-brother or sister with autism; and 2 times if they have a cousin with it. 

Autism spectrum disorder (ASD) is a neurodevelopmental disorder defined by impairments in social interaction and communication and the presence of restrictive and repetitive behaviours. The exact causes are unknown but evidence has shown it is likely to include a range of genetic and environmental risk factors.

In the study, published in the journal JAMA Today, researchers analysed anonymous data from all 2 million children born in Sweden between 1982 and 2006, 14,516 of which had a diagnosis of ASD. The researchers analysed pairs of family members: identical and non-identical twins, siblings, maternal and paternal half-siblings and cousins. 

The study involved two separate measures of autism risk: heritability – the proportion of risk in the population that can be attributed to genetic factors – and relative recurrent risk, which measures individual risk for people who have a relative with autism.  

Most previous studies have suggested that heritability of autism may be as high as 80-90%, but one study has hinted at a lower estimate. This study, which is the largest and most comprehensive to date, estimates heritability of autism to be 50%, with the remainder explained by non-heritable or environmental factors. 

Environmental factors are split into ‘shared environments’, which are shared between family members, such as family socio-economic status, and ‘non-shared environments’, which are unique to the individual – such as birth complications, maternal infections or medication during the pre and perinatal period. The study found that non-shared environments were the major source of environmental risk.

Study author Professor Avi Reichenberg, from Mount Sinai Seaver Center for Autism Research, who led the study while at King's College London, said: “Heritability is a population measure, so whilst it does not tell us much about risk at an individual level, it does tell us where to look for causes. We were surprised by our findings as we did not expect the importance of environmental factors in autism to be so strong. 

“Recent research efforts have tended to focus on genes, but it’s now clear that we need much more research to focus on identifying what these environmental factors are. In the same way that there are multiple genetic factors to consider, there will likely be many different environmental factors contributing to the development of autism.” 

Dr Sven Sandin, author of the study from King’s College London and Karolinska, added: “Our study was prompted by a very basic question which parents often ask: ‘if I have a child with autism, what is the risk my next child will too?’ Our study shows that at an individual level, the risk of autism increases according to how close you are genetically to other relatives with autism. We can now provide accurate information about autism risk which can comfort and guide parents and clinicians in their decisions.”