The years before beginning a brand-new decade—ages 29, 39, and so on—tend to be spent in self-reflection, according to a new paper published online today in Proceedings of the National Academy of Sciences. These are the prime What am I doing with my life? years, in other words, which prompts many people to behave in ways that suggest “an ongoing or failed search for meaning,” the authors write. Their data suggests that these are the ages when people are more likely to either train harder for a marathon or run one for the first time; they’re also the ages when more people tend to cheat on their marriages or take their own lives.
One way to think of the last year of a decade, in other words, is that it’s a 12-month-long transition period to a new life stage. “And transitions, they make us step back, they make us evaluate things,” said Hal Hershfield, a UCLA psychologist who co-authored this study with New York University’s Adam Alter. “Once you’re in the midst of something, it’s easier to keep your head down and just go, go, go. But when you take a step back, that’s when you say, 'Wait, let me see how everything’s going here.'”
In one study, Hershfield and Alter found that out of more than 8 million male users on an Ashley Madison–esque dating site—the kind designed for people seeking affairs—there were more than 950,000 men aged 29, 39, 49, or 59, or nearly 18 percent more than would be expected by chance. (They looked at women using the site, too, and found a similar trend, but the effect wasn’t as strong.)
Another study used Centers for Disease Control data from 2000 to 2011 to look at the suicide rate of men and women between the ages of 25 and 64, and the researchers found that the rate of suicide among those in their last year of a decade was slightly (though statistically significant) higher than the rate for any other age.
Finally, two studies used data from the website Athlinks, which tracks finishing times for marathons and other races. The researchers analyzed the times for runners who’d completed one marathon at age 29 or 39, and at least one more during the two years before and the two years after. These runners tended to finish 2.3 percent faster when they were 29 or 39 compared to the years before and after that birthday, suggesting that they trained harder for the race the year before they turned 30 or 40. The researchers also looked at 500 runners aged 25 to 64 and found that 74 of them were in their last year of a decade—that’s an over-representation of 48 percent, according to the paper.
These findings suggest that in the year before entering a new decade of life, we’re “particularly preoccupied with aging and meaningfulness, which is linked to a rise in behaviors that suggest a search for or crisis of meaning,” as Hershfield and Alter write. The meaning we ascribe to one decade ending and another beginning is another example of how much we value a “fresh start,” as recent research has also suggested.
And it also signals the psychological power of round numbers, Hershfield said. For example, high-school students who score just below a round number on the SAT are more likely to take the test over again than those who score just over a round number, a 2011 study found. And, to once again revisit the marathon, a recent analysis of 9 million marathon finishers showed the runners’ times tended to cluster around the round numbers like 4:00, 3:30, and so on, suggesting the runners pushed themselves to hit that round number. “It feels cleaner, neater,” Hershfield said, and I’d argue that a round number also gives us a sense of resolution that an odd number does not.
In future research, Hershfield would like to figure out why some people respond to periods of pre-birthday reflection in maladaptive ways, like seeking out affairs or choosing to end their lives. Likewise, a better understanding of why this self-evaluation prompts a positive response for others could help researchers figure out how to get people to adopt other healthy goals around this time, like quitting smoking or losing weight. We know that the end of a perceived era prompts us to make big life decisions; now the question is how to harness that motivation and use it for good.