To a biologist or physician, the physiological differences between, say, 39-year-old Fred and 44-old Fred aren’t vast—probably not much different than those between Fred at 38 and Fred at 39. Nor do our circumstances diverge wildly in years that end in nine compared with those that end in zero. Our life narratives often progress from segment to segment, akin to the chapters of a book. But the actual story doesn’t abide by round numbers any more than novels do. After all, you wouldn’t assess a book by its page numbers: “The 160s were super exciting, but the 170s were a little dull.” Yet, when people near the end of the arbitrary marker of a decade, something awakens in their minds that alters their behavior.