The findings of the small-scale study give hope to parents of children born with a severe intellectual disabilities who are worried about having another baby with the same condition, said Anita Rauch, a researcher at the Institute of Medical Genetics in Zurich who was one of the study's lead authors.
It examined the genetic makeup of 51 children, both of their parents and a control group. The study concluded that in at least 55 percent of cases there was no evidence that parents carried faulty genes responsible for the disability.
"The average chances of having another child with the same disability are usually estimated at eight percent, but if we know that it was caused by a random mutation the chances of recurrence drop dramatically," Rauch said.
Hans-Hilger Ropers, the director of Berlin's Max Planck Institute for Molecular Genetics, who was not involved in the study, said the basic science appeared sound but noted that it excluded children whose parents were blood relatives and so the results could be biased toward random mutations.
Ropers said a larger study that included subjects from parts of the world where marriage between blood relatives is more common could produce different results.
But he added that the study helped explain why the frequency of children born with intellectual disabilities is stable at about 2 percent. If most disabilities were inherited then they would be expected to diminish in frequency because people who suffer from them are less likely to have children.
"Since mentally handicapped people rarely have children, the frequency of the responsible gene defects would be expected to diminish over time," Ropers said. "Since there is no evidence that it does, the loss of genetic defects must be compensated by novel mutations."
The study was published online by the journal Lancet.