Many years of research, more and more cases diagnosed, and scientists still do not know what exactly causes autism in children. However, they are finding more and more risk factors. It is worth knowing which ones.

Knowledge of the causes of autism is becoming increasingly desirable and relevant to a growing number of families. Indeed, the prevalence of the disorder increased by an estimated 23 per cent between 2006-08, reads a report released last week by the Centers for Disease Control and Prevention in Atlanta.

In most cases, researchers cannot tell parents what caused their child’s autism, says Thomas Insel, director of the US National Institutes of Mental Health. Mostly, the causes of autism – which is probably not a single condition, but a group of diseases with related symptoms – remain a mystery.

For years, scientists have had only a few proven facts about the condition: for example, that it affects boys four times more often than girls. More recently, however, researchers have confirmed a number of factors that increase the risk of autism, many of which have to do with problems occurring very early in life – during pregnancy or childbirth, for example, or even during the process of egg and sperm formation, explains Craig Newschaffer, professor at Drexel University in Philadelphia.

To better understand the causes of autism, researchers from four prominent universities are tracking the health of 1,200 mothers of autistic children as part of a project called EARLI, i.e. Early Autism Risk Longitudinal Investigation. Because the researchers know that these women have a high risk of having a second child with autism, they followed their subsequent pregnancies very closely, analysing samples of blood, urine, hair and even collecting dust from their homes, says Newschaffer, one of the co-authors of the study. The researchers also asked the pregnant ladies to write down any illnesses they contracted, as infections during pregnancy are also suspected to be involved in the development of autism.

Doctors can confidently assure parents that one thing does not cause autism – vaccines, says Paul Offit, head of the infectious diseases department at Children’s Hospital in Philadelphia. More than 20 studies have found no link between autism and vaccines, given separately or in combination.

Researchers do, however, have clues about other risk factors:


About 15-20 per cent of children with autism have a genetic mutation that contributes to the development of their disorder, says Insel. Some genetically determined diseases, such as broken X chromosome syndrome or Rett syndrome, are well known to increase the risk of autism. But even if genes were a major contributor to the development of the disorder, it is possible that most children have a unique mutation or set of mutations, says David Amaral, director of research at the University of California, Davis.

Family history

If parents have one child with autism, the risk of having a second child with the disorder is nearly 20 per cent. – according to a landmark study by researchers at the University of California, Davis. For those who have two autistic children, the likelihood that a third will also be autistic is already as high as 32 per cent, comments study author Sally Ozonoff.

Environmental pollution

One Californian study published last year found that children whose mothers lived close to a motorway during pregnancy were more likely to be diagnosed as autistic.

Older parents

Both older fathers and older mothers have a higher risk of having children with autism, says Newschaffer. Research by scientists in Israel and at the Harvard School of Public Health also suggests that infertility treatments, which are more common in older patients, are associated with a higher risk of the disorder.

Prematurity and low neonatal weight

A paper published in October 2011 in the journal Pediatrics found that 5 per cent of children who weighed less than 2 kilograms at birth were diagnosed with autism before the age of 21.


A number of studies indicate that the anticonvulsant drug valproic acid may increase the risk of autism in children who were exposed to it before birth. Another study published last year found a higher risk of the disorder among children exposed in fetal life to anti-depressants. In contrast, prenatal use of vitamins has been linked to a lower risk of autism.

Pregnancies in quick succession

A 2011 study found that children who were born less than a year after an older sibling were diagnosed with autism three times more often than toddlers who were born three years after their mother’s last pregnancy.