Still, helmets are not the problem, behavior is. Having a helmet allows (and football coaches teach) styles of tackling and blocking that would be impossibly dangerous for the bareheaded. By contrast, ruggers use their arms and shoulders to make contact, keeping their heads clear. Contact above the shoulders in rugby is a penalty; more dangerous plays earn you a yellow card and 10 minutes in the “sin bin,” forcing your team to play short-handed.

In rugby, I was a “prop,” spending my afternoons with one arm around a sturdy little hooker. (Less racy than it sounds — hookers are the players who contest the ball in a scrum.) I never saw anyone get his “bell rung.” In football, I played offensive tackle, and saw at least five cases of bell-ringing in games. We’d ask the guys what was the name of the team we were playing against, and they would talk about fishing, or their grandmother.

There are three things going on in football, and it’s important to keep them separate. The first is the formal rules, which attempt to limit concussions. The second is conventional tackling practice, which has a high risk of concussion. And the third is the informal rules, or “the code.” The sportswriter Ross Bernstein discussed this in his book “The Code,” quoting the former N.F.L. offensive lineman Dan Dierdorf:

Your goal is to dominate that guy … There are right ways and wrong ways of doing that, however, and that is what the code is all about. I never once tried to injure the guy across from me … but wanted to send him to the bench — and there’s a difference.

This “difference” might strike you as awfully thin. How do you distinguish between trying to injure someone and trying to hurt him? But to anyone who has played football or rugby, it makes perfect sense. If you tackle someone cleanly and happen to snap his rib, then good-on-ya (as the Aussies would say). But if you go after a guy’s knee, even your own teammates won’t defend you. You are a bad guy.

When formal rules and the informal norms of sports conflict, players (and the game) suffer. In football today, the rules (no head shots) and norms (head shots are part of the game) conflict. And then there’s the other factor, tackling practice: Almost everyone believes that the helmet-first tackling style is more effective. As Dierdorf said, sending a man to the bench has been a badge of honor, not a violation of the code, even if you intended to knock him out. Anyone who avoids delivering a blow to avoid ringing the guy’s bell is a wimp, and he also risks missing the tackle. Formal rules will never be enough to deter head shots under those conditions.