Obviously, controlled experiments can't be conducted in this realm.

Well no, not on humans at least. But the medical profession seem to be pretty good at extrapolating data derive from animal experiments. Whilst these may be controversial now, back in the 40s, 50s and 60s when most of the information relating to radiation effects was first being discovered and catalogued, there were fewer concerned about such things, but in the limited reading I did, I found very little reference to long term studies of that type.

Maybe they were done before electronic media was commonplace and so have never made the transition. Even so, you might expect to find some references. Or maybe they were all classified and haven't reached their declassification dates yet. Or maybe I just didn't look hard enough, or in the right places.