BERLIN (AP) — More than half the cases of severe intellectual disability caused by genetic defects are the result of random mutations, not inherited, a European study published Thursday suggests.

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.