Epidemiology of Autism - Risk Factors

Risk Factors

Boys are at higher risk for autism than girls. The ASD sex ratio averages 4.3:1 and is greatly modified by cognitive impairment: it may be close to 2:1 with mental retardation and more than 5.5:1 without. Recent studies have found no association with socioeconomic status, and have reported inconsistent results about associations with race or ethnicity.

Although the evidence does not implicate any single pregnancy-related risk factor as a cause of autism, the risk of autism is associated with several prenatal risk factors, including advanced age in either parent, and diabetes, bleeding, and use of psychiatric drugs in the mother during pregnancy. It is not known whether mutations that arise spontaneously in autism and other neuropsychiatric disorders come mainly from the mother or the father, or whether the mutations are associated with parental age. However, recent studies have identified advancing paternal age as a significant risk factor for ASD.

A large 2008 population study of Swedish parents of children with autism found that the parents were more likely to have been hospitalized for a mental disorder, that schizophrenia was more common among the mothers and fathers, and that depression and personality disorders were more common among the mothers.

It is not known how many siblings of autistic individuals are themselves autistic. Several studies based on clinical samples have given quite different estimates, and these clinical samples differ in important ways from samples taken from the general community.

Autism has also been shown to cluster in urban neighborhoods of high socioeconomic status. One study from California found a three to fourfold increased risk of autism in a small 30 by 40 km region centered on West Hollywood. Another study by a UC Davis group using a similar methodology,erroneous because of its stratification of space nevertheless found multiple clusters in urban high socioeconomic status neighborhoods of California.