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First-born children of women older than 35 and men older than 40 are three times more likely to develop autism than later children of younger parents, according to a large epidemiological study in the United States.

This is not the first study to make this connection: smaller epidemiological studies have found a link between older parents and greater risk of autism, schizophrenia and, most recently, bipolar disorder.

For this study, published in the American Journal of Epidemiology, researchers scanned the records of 253,347 children born in 10 states ― part of the federal Autism and Developmental Disabilities Monitoring Network ― in 1994. Of these children, 1,251 were later diagnosed with autism.

The researchers found that for every 10-year increase in the motherʼs age, the risk of autism jumps up 20 percent; for men, every 10-year increase in age adds up to a 30 percent higher risk of autism in the child.

Itʼs not entirely clear why this should be the case, but as men and women age, they are presumably more likely to acquire and pass on spontaneous mutations and chromosomal changes to their children.

Intriguingly, the study also suggests that the risk of autism decreases with birth order. The researchers say the trend in Western countries for people to have the first child later in life, and to have fewer children ― meaning first-borns are a greater proportion of children ― may in part explain the rise in autism prevalence.