How many brilliant people do you know who are nonetheless capable of making ridiculous mistakes? Maybe that’s even you. The thing is, intelligence is complicated. There are different kinds: Someone who’s a genius at music maybe be a total washout in social circumstances, a brilliant philosopher may not be able to remember actors’ names, and so on.
There are tests with which we measure different types of intelligence, and the IQ test towers over them all. “So-and-so has an IQ of this or that,” we hear, as if it's (a) a literal measurement of that person’s mental potential, and/or (b) a fixed value that can never change. In fact, this isn’t how the tests work at all.
It seems like most people only get tested once, at school, and then carry their IQ scores as badges of pride — or embarrassment — the rest of their lives. But, as David Shenk says, for those who do get re-tested and remain in the same percentile, it doesn’t mean the entire percentile hasn’t gotten smarter — nor does it mean it has. It just means it’s still in the same position relative to the rest of the tested population. And downward movement doesn’t mean you’ve gotten dumber in your old age.