From The Wall Street Journal, "Jenkins: Nascar's Creative Destruction: Here's one great business innovation no one seems to want to copy." by Holman W Jenkins:
As with football, though, participant risk-taking tends to increase in tandem with safety. In a comparison noted by many, rugby is a brutal and aggressive sport, and players don't wear helmets. Rugby players, therefore, don't use their heads as weapons, and they play in such a way as to spare their craniums.