Suppose the commissioner of the national football league (nfl) institutes a rule requiring players to wear a new, lighter helmet that is designed to reduce head trauma during collisions between players on the field. while the new helmets the probability of a concussion resulting from each individual collision, at the same time, the new helmets could also give football players an incentive to play more which could potentially the amount of violent collisions and thus the number of concussions suffered by nfl players, all else equal.