The year before we age into a new decade — at 29, 39, 49 and so on — we’re more likely to back up and take a hard look at our lives. Year nine, that time leading up to a new chapter, can prompt us to take action, making us more likely to run a marathon, cheat on our partners or even take our own lives, according to research published this week in Proceedings of the National Academy of Sciences.

As one chapter closes and another one opens, research suggests, it’s human nature to start searching for something.

“Years 30, 40, 50, they’re psychologically consequential,” Hal Hershfield, a psychologist at the University of California at Los Angeles and co-author of the study, told The Washington Post in an interview. “They seem big, they seem looming and they seem more important to us than the others. They make us step back and think about how things have been going up until then and how we want them to go moving forward.”

The reason? We place arbitrary importance on numbers.

“Round numbers are salient in society, so they become strong motivators of behavior,” Hershfield said. That’s why high-schoolers who score shy of a round number on the SAT are more likely to take it again, and professional baseball players hustle to finish the season with a batting average just above rather than below .300.

The authors looked at six studies, analyzing the psychological and emotional changes that occur at this age, when people are “nine-enders,” approaching new decades in life.

The researchers examined data from more than 42,000 adults in more than 100 countries who took the World Values Survey, which asks people to assess their lives. Hershfield said they found people who were “in the nines” were more likely to question whether their lives held meaning.

“People are more likely to search for meaning at that time,” Hershfield said. “And we can essentially approach that search for meaning in adaptive ways or maladaptive ways.”

For instance, the authors collected data from Athlinks, a site that records running times. Some 25 percent more nine-enders ran marathons for the first time than people of other ages. In addition, runners ages 29 and 39 who had run multiple marathons ran about 2 percent faster at that age than they did during other years.

Alternatively, the researchers looked at destructive behaviors. They examined data on more than 8 million men registered on the dating site Ashley Madison, which caters to people looking for extramarital affairs. They found that men claiming to be 29, 39, 49 and 59 were almost 18 percent more likely than men of other ages to register on the site. However, Hershfield noted that some people on the site may lie about their ages to appear younger.

“We deal with this in two ways,” he said via e-mail. “We asked several hundred people what age they would lie to if they were trying to deceive someone on a dating Web site and we found no evidence that people specifically lied about an age ending in nine. And if there is any pressure to lie about being younger on the dating Web site, then there’s also pressure to lie about being older for marathons. So, the fact that we continue to find the same effect, even despite opposing incentives to lie, suggests the effect is legitimate.”

They also analyzed the Centers for Disease Control and Prevention’s data on the national suicide rate from 2000 to 2011 and discovered that instances were 2.4 percent higher among people whose ages ended in nine than any other digit.

“There’s a little bump at the nines,” Hershfield said. “Our effects are not huge and, in a way, we would not expect them to be. There are a lot of factors that go into these decisions. But the fact that we saw any bump was significant for us. When you’re talking about things like suicide, that’s a big deal. It might be numerically small but it’s practically significant.”