Some book I read as a kid, possibly Mathematics for the Million by Lancelot Hogben, said that prehistorical humans could count up to three, and after that it was just “many.” I don’t know if there is any actual evidence for this, and it’s always struck me as fishy since prehistorical people had more than three of many things, including children. And, sure, there are ways to track stuff without counting — old timey shepherds would take a pebble for each sheep when they left in the morning so they could tell if they were coming back with the same number — but why 3 and not 4 or 11?
But our modern rhetoric retains a trace of this “more than three is many” idea.
Imagine you are writing an essay about what’s the last thing you’d like to see before you go blind, or what musician affects you more than any other. If you choose to structure your essay by beginning with choices you’ve contemplated but rejected, you have to list three. If you only list two, it will seem like you have singled them out as the only two competitors; two signals completeness. If you list three, you’re indicating that there are in fact many more you could discuss. For example: “Jazz is too free-form. Hip-hop is too talk-y. Folk is too kumbiyah. But show tunes…well, show tunes have it all going on.”
Notice, by the way, that the third in the list is an opportunity for a joke, because the basic structure of a joke is two + punchline. It’s “A Brit, a German, and a Jew walk into a bar,” not “A Brit, a German, a French guy, a Belgian, a Swede, a Pole, a Laplander, a guy with a parrot, and a Jew walk into a bar.” I believe this is because you need two to claim that it’s a pattern, but since the pattern itself is not amusing, adding more examples only annoyingly delays the disruption of the pattern by the third. So,joke = two + punchline.
Hey, I don’t make the rules.