 -- People whose ages end in 9 tend to be more likely to seek extramarital affairs, run marathons and commit suicide compared with those whose ages ended in other digits, according to a new study.

Researchers at New York University's Stern School of Business and University of California's Anderson School of Management conducted six studies to see how people in the last year of their 20s, 30s, 40s and 50s changed their behavior. They found that people they've nicknamed "9-enders" -- people who were 29, 39, 49 or 59 -- were more likely than others to reflect on their lives and make big changes, according to the study published in the journal Proceedings of the National Academy of Sciences.

"When people are facing these new decades, that's when they start to step back and question essentially the meaningfulness of their lives," said study co-author Hal Hershfield, a marketing professor at UCLA who was trained as an experimental social psychologist. "We're not saying people don't do that at other points in their lives. Just that it's particularly likely to happen during life transitions."

Hershfield and his co-author Adam Alter came up with the idea for their study while discussing greeting cards and the big deal people make around entering new decades of their lives.

"It's not like anything officially changes," Hershfield said. "It's not like you got married or you can drive now or you're Bar Mitzvahed."

Yet they wanted to study how much meaning is attached to these milestones, particularly for people about to cross into their 30s, 40s, 50s or 60s. So they used data from the U.S. Centers for Disease Control and Prevention as well as data from extramarital affairs site AshleyMadison.com and athlete site Athlinks.com.

Chief of psychology at University Hospitals Case Medical Center Jeff Janata, who was not involved in this research, called the study "clever" because it uses actuarial data to examine a "psychological truth."

"I think that people use decades and the crossing from one decade into the next as a marker, a time to reflect on the state of their lives. I think it's very common," he said. "What we're really talking about is anticipation more than we are arrival."

On the one hand, Hershfield and Alter reasoned that people could react negatively to their impending milestone birthdays by committing suicide or seeking extramarital affairs. On the other, they could set a healthy goal, like running a marathon. They found 9-enders were more likely to do all of these.

And 9-enders ran faster marathons than people two years older or younger than they were, proving they trained harder, according to the study.

"A lot of different factors go into the decision to run marathon, commit adultery or end one's life," Hershfield said. "We wouldn't expect just facing down the barrel of their 40s, 50s would be enough to change it drastically, but it changes it somewhat enough that we could pick up on it statistically."

His co-author, Alter, said he hopes the study gives casual readers pause to think about why they're making the changes in their lives.

"In general, it's easy to get caught up in big milestones, particularly as we age -- but of course there’s no real difference between turning 30 and turning 29 or 31," he said. "Our culture emphasizes years like 30, 40, 50, and 60, but we shouldn't let that shape how we live our lives."