Milestone birthdays ending in the number 9 -- when we reach the end of another decade of life -- tend to push people toward making significant decisions about their lives, for better or worse, researchers have found.
In that last year of a life decade, people are more likely to engage extramarital affairs, decide to start running marathons and even choose to commit suicide, compared with those whose ages ended in other digits, a study indicates.
Analyzing how people in the last year of their 20s, 30s, 40s and 50s changed their behavior, it was found that people nicknamed "9-enders" -- people aged 29, 39, 49 or 59 -- were considerable more likely than others to reflect on life situations and decide to make big changes, the study appearing in the journal Proceedings of the National Academy of Sciences found.
"People audit the meaningfulness of their lives as they approach a new decade in chronological age," says study lead author Adam Alter from New York University.
Although often seen in people in their 40s and 50s -- the all-too-familiar "midlife crises" -- Alter and his co-researcher Hal Hershfield from the University of California, Los Angeles, say they believe people tend to make significant changes as they advance toward each new decade, not just in their middle years.
"When people are facing these new decades, that's when they start to step back and question essentially the meaningfulness of their lives," says Hershfield, a marketing professor 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."
In an attempt to gauge how adults might initiate a contemplation of existential meaning as they near a new life decade, and how that might affect their behavior, Alter and Hershfield looked at a number of existing studies that measured the incidence of extramarital affairs, exercise and suicide rates among adults between the ages of 25 and 64.
What they found was evidence that nine-enders "reported questioning the meaning or purpose of life more than respondents whose ages ended in any other digit," the researchers said.
The imminent arrival of a new decade of life provides the opportunity for a beneficial reassessment of our priorities and an examination of our behavior as we age, they suggest.
Still, they say, we shouldn't get too hung up on any particular birthday.
"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," Alter says. "Our culture emphasizes years like 30, 40, 50, and 60, but we shouldn't let that shape how we live our lives."