Most cases of severe intellectual disability are not inherited, but are instead the result of random genetic mutations, a new study suggests.

The finding should offer reassurances to parents of those with intellectual disability who are considering having more children that the likelihood of passing on the condition is low, researchers said.

While some genes have been identified as causing intellectual disability, it’s unclear what leads to the condition in most people. But in a new study published online Thursday in The Lancet, European researchers found that in as many as 55 percent of cases, gene mutations were responsible.

For the study, scientists looked at 51 children from Germany and Switzerland who have intellectual disability with no known cause. They examined the genetic makeup of the kids and their parents as well as a control group.

In more than half of cases, there was no evidence of any genetic abnormality in the children’s parents responsible for development of the disability.