All four of them were what the social psychologists Adam Alter and Hal Hershfield call “nine-enders,” people in the last year of a life decade. They each pushed themselves to do something at ages 29, 39, 49, and 59 that they didn’t do, didn’t even consider, at ages 28, 38, 48, and 58—and didn’t do again when they turned 30, 40, 50, or 60.

Of all the axioms describing how life works, few are sturdier than this: Timing is everything. Our lives present a never-ending stream of “when” decisions—when to schedule a class, change careers, get serious about a person or a project, or train for a grueling footrace. Yet most of our choices emanate from a steamy bog of intuition and guesswork. Timing, we believe, is an art.

In fact, timing is a science. For example, researchers have shown that time of day explains about 20 percent of the variance in human performance on cognitive tasks. Anesthesia errors in hospitals are four times more likely at 3 p.m. than at 9 a.m. Schoolchildren who take standardized tests in the afternoon score considerably lower than those who take the same tests in the morning; researchers have found that for every hour after 8 a.m. that Danish public-school students take a test, the effect on their scores is equivalent to missing two weeks of school.

Other researchers have found that we use “temporal landmarks” to wipe away previous bad behavior and make a fresh start, which is why you’re more likely to go to the gym in the month following your birthday than the month before.

Chronological decades have little material significance. 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.

For example, to run a marathon, participants must register with race organizers and include their age. Alter and Hershfield found that nine-enders are overrepresented among first-time marathoners by a whopping 48 percent. Across the entire lifespan, the age at which people were most likely to run their first marathon was 29. Twenty-nine-year-olds were about twice as likely to run a marathon as 28-year-olds or 30-year-olds.

Meanwhile, first-time marathon participation declines in the early 40s but spikes dramatically at age 49. Someone who’s 49 is about three times more likely to run a marathon than someone who’s just a year older.