Why autism strikes certain kids -- and not others -- has puzzled scientists for half a century. But one trend is clear: having older parents seems to increase the risk. Now, a study from Drexel University adds two more wrinkles to the story.

Brian Lee, a professor of epidemiology at Drexel University's School of Public Health, analyzed a Swedish database of more than 400,000 births to see how the ages of both parents related to a child's chances of autism.

"The mothers' risk seems to be, quantitatively speaking, more important than the father's," he said. "Having a mother who is 40 is going to have a bigger effect on the child's autism risk than having a father who is 40 years old."

The oldest mothers studied, ages 40 to 45, were 75 percent more likely than younger women to have kids with an autism spectrum disorder. Fathers of similar age had just a 14 percent added risk. How risk increased with age differed between parents, accelerating for moms but climbing in step for dads.

Lee also examined the interaction of both parents' age, finding older fathers increased risk only with younger mothers.

"The effect of the father seems to essentially be washed out when you have older mothers," he said.

Lee said the results aren't meant to discourage older couples from having children. Less than 1.9 percent of the oldest women in the study gave birth to children who would go on to develop autism.

"The reason we're interested in parental age in the first place is because it's a marker for some underlying mechanism that really we're interested in," he said.

According to Lee, knowing age seems to affect moms and dads differently opens up new avenues of investigation.