(CBS News) The cause of autism is one of the great mysteries of medicine, but we found out something new and important Wednesday.

A report in the medical journal "Nature" says it is the age of the father at the time of conception, not the age of the mother that can raise the risk of autism in a child.

With more men becoming fathers later in life, it could help explain the rise in autism.

The study's senior author - a scientist in Iceland - writes: "Conventional wisdom has been to blame the developmental disorders of children on the age of mothers"... but "it is the age of fathers that appears to be the real culprit."

Dr. Delores Maldespina, a psychiatrist at New York University, said she finds the research eye opening.

"This study shows that new mutations are frequent enough as men age that fathers aging alone can explain the increase for the risk of autism," Maldespina said.

The study says a father's age could account for 15 to 30 percent of cases of autism, and perhaps other disorders like schizophrenia as well.

"This study shows that when these diseases present without a family history, the origin is in the sperm of the man and that the risk goes up as the man ages," Maldespina said.

The latest numbers from the Centers for Disease Control indicate the rate of autism has doubled in the last six years from 1 in 162, to 1 in 88.

The number of men 40 and older who father a child has increased by more than 30 percent since 1980. The study is the first to quantify the consequence of a father's age: For every 1-year of age, two genetic mutations were discovered in offspring that could be traced to the father, and up to 65 mutations in the offspring of 40-year-old men. The average number of mutations traced to the mother was 15, no matter what her age.

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Study: Father's age a likely factor in autism