The New York Times reported that according to scientists, older men are more likely than young ones to father a child who develops autism or schizophrenia, because of random mutations that become more numerous with advancing paternal age. The age of mothers had no bearing on the risk for these disorders, the study found.
Experts said that the finding was hardly a reason to forgo fatherhood later in life, though it might have some influence on reproductive decisions. The overall risk to a man in his 40s or older is in the range of 2 percent, at most, and there are other contributing biological factors that are entirely unknown.
However, the study, published online in the journal Nature, provides support for the argument that the surging rate of autism diagnoses over recent decades is attributable in part to the increasing average age of fathers, which could account for as many as 20 to 30 percent of cases.
The findings also counter the longstanding assumption that the age of the mother is the most important factor in determining the odds of a child having developmental problems. The risk of chromosomal abnormalities, like Down syndrome, increases for older mothers, but when it comes to some complex developmental and psychiatric problems, the lion’s share of the genetic risk originates in the sperm, not the egg, the study found.
Previous studies had strongly suggested as much, including an analysis published in April that found that the risk factor stemming from sperm was higher at age 35 than 25 and crept up with age. The new report quantifies that risk for the first time, calculating how much it accumulates each year.
The research team found that the average child born to a 20-year-old father had 25 random mutations that could be traced to paternal genetic material. The number increased steadily by two mutations a year, reaching 65 mutations for offspring of 40-year-old men. The average number of mutations coming from the mother’s side was 15, no matter what her age, the study found.