Sometimes wonder whether the hard outer shell of football helmets, in spite of the very elaborate internal cushioning, contributes to the brain trauma? Rugby is also a rough collision sport but the helmets players (optionally) wear are light weight cushioned, not unlike the head gear you used to see in photos of old time Soviet tank crews. When I played football in high school I remember wearing the helmet made me feel almost invulnerable. But at the college or grown man professional level of size and speed the collisions are akin to being hit by a bus. There are probably legitimate tests that show the hard shell with internal cushioning (like a motorcycle helmet) is safer, but I’m just speculating.