Genes may play a bigger role in cerebral palsy than previously thought with researchers finding heightened risk in siblings and other relatives of those with the condition.

In a study looking at more than 2 million children born in Norway between 1967 and 2002, researchers found that the likelihood of cerebral palsy appears greater among those most closely related to individuals with the developmental disability.

Twins face the highest risk. When one child has the condition, their twin has a 15 times greater chance of also being affected. Meanwhile, full siblings face a six to ninefold increased risk and half-siblings of those with cerebral palsy are up to three times more likely to have it too, according to findings published this week in the British Medical Journal.

The elevated odds also extend to parents with cerebral palsy who are 6.5 times more likely than those who are unaffected to have a child with the condition. Cousins face a 1.5 times increased risk, the study found.

Cerebral palsy has previously been linked to several factors including premature birth, infection, abnormal growth and lack of oxygen at birth, but researchers said this is the first study of its size to consider how the condition runs in families.

“Our data suggest that cerebral palsy includes a genetic component, with a stronger recurrence among relatives with closer genetic relationship,” the team of Norwegian and American researchers wrote in their findings. “This offers additional evidence that the underlying causes of cerebral palsy extend beyond the clinical management of delivery.”

In an accompanying editorial, Peter Rosenbaum of McMaster University said much remains to be understood about cerebral palsy and that even among twins the risk of recurrence remains relatively small.

“The elusive search for the causes of neurodisabilities such as cerebral palsy is far from over,” Rosenbaum wrote. “While we wait, however, we must remember that cerebral palsy remains a relatively infrequent occurrence at about 2 per 1,000 live births in developed countries.”