There's a reason we feel that mixture of dread, excitement, and existential restlessness right before we tip over into the next decade—and scientists say that can be both good and bad. According to a study published in Proceedings of the Natural Academy of Sciences, people turning an age that ends in 9 are more likely to take up positive habits, but they may also cheat on their mates and even be more apt to commit suicide, the Washington Post reports. That's because the approaching round-numbered ages (e.g., the big 3-0 or 4-0) are "psychologically consequential" and cause us to take a long, hard look at what we've been doing up to that point and recalibrate, Hal Hershfield, one of the study's co-authors, tells the Post. People want a do-over when they hit certain temporal landmarks like birthdays or New Year's Day—what a study published in Management Science calls the "fresh start effect."
In the new research, scientists analyzed six studies and found that people the Post refers to as "nine-enders" are more likely to question how meaningful their lives are, and are more likely to increase both constructive and destructive behaviors, the NYU Stern School of Business reports. Men who were 29, 39, 49, and 59, for example, were more likely to register on an extramarital affair-focused website, while suicide rates for nine-enders were 2.4% higher than for others. On the flip side, first-time marathon runners were more likely to have an age ending in 9 (25%), and runners who were 29 and 39 had a 2% faster running rate than they did two years before and after those ages. The research underscores the powerful psychological pull that numbers can wield. (Death rates overall seem to spike on birthdays.)