New research finds a lot of people who are approaching a new decade in life ponder the meaning of existence, and go looking for answers.

As we all know, the urge to find meaning in life can inspire both good or bad choices. But what sets off that existential yearning? For some, it's a dramatic event that shakes us out of our complacency, such as the death of a loved one. For others, it's a nagging feeling that there must be something more.

But for a lot of folks, it's a number. Specifically, one that ends in a nine.

New research suggests that people who are approaching a new decade in life—that is, individuals whose current age is 29, 39, etc.—are more likely than those of other ages to ruminate about the purpose of their lives. What's more, they also appear to be more willing to try new experiences, ranging from running a marathon to having an extramarital affair.

Most disturbingly, according to a paper published in the Proceedings of the National Academy of Sciences, they're also at heightened risk for committing suicide.

At least in the U.S., "the suicide rate was higher among 9-enders than among people whose ages ended in any other digit."

"People audit the meaningfulness of their lives as they approach a new decade in chronological age," write Adam Alter of New York University and Hal Hershfield of the University of California-Los Angeles. Over a series of studies, they found "9-enders" are particularly prone to behaviors "that suggest a search for, or a crisis in meaning."

One such study featured 42,063 adults from more than 100 countries who completed the World Values Survey. They reported on a scale of one to four ("never" to "often") how often they questioned the meaning and purpose of life.

Nine-enders "reported questioning the meaning or purpose of life more than respondents whose ages ended in any other digit," the researchers write.

For another study, Alter and Hershfield analyzed data on male users of "a dating website that caters to people who are seeking extramarital affairs." They found "952,178 9-enders registered on the site, 17.88 percent more than if the frequency of end-digits were randomly distributed."

Another study found nine-enders "tend to be overrepresented among first-time marathon runners." They randomly chose 500 first-time participants in one of five major marathons in the U.S. Of that sample, "74 were 9-enders, an overrepresentation of 48 percent."

On a more somber note, they also report that, at least in the U.S., "the suicide rate was higher among 9-enders than among people whose ages ended in any other digit." Alter and Hershfield came to this conclusion using regional data from the Centers for Disease Control and Prevention.

The researchers note that some of the effects they found were small. But together, they suggest the approach of a new decade of life can serve as a wake-up call—one that can lead to a thoughtful re-assessment of our priorities, or inspire problematic attempts to reclaim our lost youth.

By the way, I'm a nine-ender myself, but have yet to engage in any of the aforementioned behaviors. I don't expect that will change, but check back with me in six months.