Is Head Impact a Cause of CTE?

It seems natural to assume a sport that involves some sort of impact is going to cause serious head damage. But why is there a debate about whether it was causal or not? Boxing's obviously the most obvious one, and you wonder what boxing can do to ever change that. In sports like NFL, rugby league, I'd imagine, Ozzy football, all that kind of stuff,. There is obviously ways of preventing that. And that's probably the case for rugby.