If your economics class was graded on a curve and everyone agrees to study only half as much, everyone would get the same grade that they otherwise would earn. You, however, will earn an A if you study more than the others, a C if you study the same amount as others, and an F if everyone else studies more than you. You don't like studying, but you'd rather study and get an A than get a C without studying, or study and get a C than get an F without studying. All the students in your class get together and agree not to study but have no way of verifying if anyone does study. What is it in your best interest to do? not study
randomize over studying and not studying
drop the class
study

Because of the violent nature of the sport, professional american football players are at risk of suffering concussions (or brain injuries) during the violent collisions that occur between players during the game. violent collisions involving hits to the head are particularly risky. 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.