Not long ago, Mark Chiverton, a 33-year-old in the U.K., noticed he was making a lot of silly mistakes. He’d mix up words when writing emails, or blank on a basic term while talking to his wife. None of these slip-ups were all that concerning on their own—but they were happening frequently enough that Chiverton worried he was, to put it bluntly, “getting dumber.” “At first I thought, ‘Maybe it’s just general aging, or maybe I bashed my head and didn’t realize it,’” he says.