U. S. researchers revealed on Monday that older mothers are more likely to have a child with autism than younger ones, citing that older fathers further extend their contribution to the risk of the disorder, than when the partners are under 30.

The researchers claim to have discovered that a 40-year-old woman's risk of bearing a child later diagnosed with autism was 50 percent greater than that of a woman between 25 and 29.

However, being an older father of age 40 or older, contributes significantly to autism risk when the mother is fewer than 30.

In the study published online on Monday in the journal Autism Research, the researchers observed nearly five million births in California over the 1990s, and 12,159 cases of autism diagnosed in those children, with an aim to analyze how the risk of autism was affected when one parent was a specific age and the other was the same age or considerably older or younger.

However, the study claims are in contradiction to a 2006 study of children born in Israel that claims paternal age played a much larger role.

An author of that study, Dr. Dolores Malaspina, a psychiatrist at New York University Langone Medical Center, revealed on Monday that mothers and fathers were usually of same age that small statistical variations could appear to shift the effect of advanced age from one parent to another.