Reaching our 30s, 40s, and 50s can suddenly overwhelm us with an urge to find the meaning of life. A milestone birthday can put us at the cusp of making both good and bad choices, since we tend to seek ways to feel better about ourselves, sometimes at any expense. A recent study published in Proceedings of the National Academy of Sciences found those who have an age ending in nine — 29, 39, 49 — also known as “nine-enders” are more likely to engage in productive behaviors like running a marathon and socially damaging behaviors like having an extramarital affair.

“People audit the meaningfulness of their lives as they approach a new decade in chronological age,” said Adam Alter, lead author of the study from New York University and his colleague, Hal Hershfield of the University of California, Los Angeles. This is commonly seen in people in their 40s to 50s, known as a midlife crisis, when they begin to dwell on discontents, whether they’re real or imaginary, and fantasize about escape. However, Alter and Hershfield believe people are prone to making significant changes, even before a midlife crisis, as they approach each new decade.

The researchers sought to show how adults undertake a search for existential meaning as they approach a new decade in age, or imagine entering a new epoch, and its effects on their behavior. In six studies, Alter and Hershfield looked at exercise, extramarital affairs and suicide rates among adults between the ages of 25 and 64. This helped the researchers determine the association between numerical ages and self-reflection.

In the first two studies, over 42,000 adults from more than 100 countries completed the World Values survey, a large-scale questionnaire about values, between 2010 and 2014. They reported on a scale of one to four (“never” to “often”) how often they questioned the meaning and purpose of life. People who were entering a new decade in their lives were more likely to question whether their lives were meaningful. Nine-enders “reported questioning the meaning or purpose of life more than respondents whose ages ended in any other digit,” the researchers wrote, PS Mag reported.

The third study focused on the link between numerical ages approaching a new decade and negative behaviors, like cheating. Over eight million male users registered on a dating website that caters to people seeking extramarital affairs were categorized by age. They found there were far more nine-ender men — 18 percent more — than would be expected by chance. A similar but less-pronounced pattern was seen in women.

Consequently, Alter and Hershfield delved into the percentage of nine-enders who committed suicide for the fourth study. Using regional data from the Centers for Disease Control and Prevention, the researchers found suicide rates were 2.4 percent higher among those whose age ended in a nine than among people whose ages ended in any other digit. The nine-enders were at most heightened risk than any other group.

The last two studies, five and six, focused on “productive meaning-seeking behaviors” such as exercise. Dara from Athlinks, a website that compiles running race times, was obtained to learn about the speed of runners. The data showed runners ran about two percent faster at ages 29 and 39 than during the two years before and after those ages.

These fast runners may also be first-time runners when it comes to marathons, according to the sixth and final study. The duo observed the ages of 500 first-time marathon runners randomly drawn from Athlinks. They found 25 percent more nine-enders than runners whose ages ended in any other digit. These last two findings suggest they were more motivated and trained harder as they approached their milestone birthday.

Although some of the effects found were small, together they suggest the approach of a new decade of life could serve as a much needed wake-up call to reassess our priorities and evaluate our behavior as we age. “Our research suggests that people who are nearing the end of a decade may be more likely to make large purchases (e.g., buy life insurance, invest in retirement savings, pursue cosmetic surgery, etc.). Being aware of the tendency to do so can help consumers decide if they are making such decisions for the right reasons,” wrote the researchers, according to the press release.

Milestone birthdays may help us grasp reality better as we reevaluate our accomplishments, goals, and our lives.

Source: Alter AL and Hershfield HE. People search for meaning when they approach a new decade in chronological age. PNAS. 2014.