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With the World Cup comes the quadrennial ritual in which Americans try to redesign and improve the rules of soccer. As usual, it’s a bad idea to redesign something you don’t understand—and indeed, most of the proposed changes would be harmful. What has surprised me, though, is how rarely anyone explains the rationale behind soccer’s rules. Once you understand the rationale, the rules will make a lot more sense.

So here’s the logic underlying soccer’s rules: the game is supposed to scale down, so that an ordinary youth or recreation-league game can be played under the exact same rules used by the pros. This means that the rules must be designed so that the game can be run by a single referee, without any special equipment such as a scoreboard.

Most of the popular American team sports don’t scale down in this way. American football, basketball, and hockey — the most common inspirations for “reformed” soccer rules — all require multiple referees and special equipment. To scale these sports down, you have to change the rules. For example, playground basketball has no shot clock, no counting of fouls, and nonstandard rules for awarding free throws and handling restarts—it’s fun but it’s not the same game the Lakers play. Baseball is the one popular American spectator sport that does scale down.

The scaling principle accounts for soccer’s seemingly odd timekeeping. The clock isn’t stopped and started, because we can’t assume a separate timekeeping official and we don’t want to burden the referee’s attention with a lot of clock management. The time is not displayed to the players, because we can’t assume the availability of a scoreboard. And because the players don’t know the exact remaining time, the referee gives the players some leeway to finish an attack even if the nominal finishing time has been reached. Most of the scalable sports lack a clock — think of baseball and volleyball — but soccer manages to reconcile a clock with scalability. Americans often want to “fix” this by switching to a scheme that requires a scoreboard and timekeeper.

The scaling principle also explains the system of yellow and red cards. A hockey-style penalty box system requires special timing and (realistically) a special referee to manage the penalty box and timer. Basketball-style foul handling allows penalties to mount up as more fouls are committed by the same player or team, which is good, but it requires elaborate bookkeeping to keep track of fouls committed by each player and team fouls per half. We don’t want to make the soccer referee keep such detailed records, so we simply ask him to record yellow and red cards, which are rare. He uses his judgment to decide when repeated fouls merit a yellow card. This may seem arbitrary in particular cases but it does seem fair on average. (There’s a longer essay that could be written applying the theory of efficient liability regimes to the design of sports penalties.)

It’s no accident, I think, that scalable sports such as soccer and baseball/softball are played by many Americans who typically watch non-scalable sports. There’s something satisfying about playing the same game that the pros play. So, my fellow Americans, if you’re going to fix soccer, please keep the game simple enough that the rest of us can still play it.