“As with war, suppressing reactive aggression and following rules are fundamental to most sports. Indeed, sports might have evolved as a way to teach impulse control along with skills useful for hunting and controlled proactive fighting. What is more unsportsmanlike than punching an opponent who scores a goal or, even worse, punching a teammate who scores instead of you? Professional tennis players aren’t even allowed to say rude things on court. Surely other hominins including Neanderthals engaged in play, but I hypothesize that sports evolved when humans became self-domesticated. As noted above, it is primarily among domesticated species that adults play, and among the many reasons humans in every culture play sports, one is to teach cooperation and learn to restrain reactive aggression. Regardless of whether you are trying to beat your opponent to a pulp in a cage or impress the judges of a synchronized swimming competition, to be a “good sport” you have to play by the rules, control your temper, and get along with others. Sports also foster habits like discipline and courage that are crucial for proactive aggression such as warfare. Perhaps the Battle of Waterloo really was won on the playing fields of Eton.”