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An affair with the office spouse, a red convertible, maybe a Harrison Ford earring: They are the hallmarks of midlife crisis, when big, bad life decisions happen.

But new research busts the notion that we make our worst choices only midway through our lives, suggesting that we make our biggest decisions, good and bad, in the years preceding new decades in our lives – ages 29, 39, 49 and so on. Whether it's adultery or running a marathon, major life choices happen throughout our lives ahead of big milestones (or at the end of eras, if you prefer), according to a paper co-authored by marketing professors at New York University and the University of California, Los Angeles.

These are self-reflective years when people "audit" their lives, searching for meaning in productive and destructive ways. "Adults undertake a search for existential meaning when they approach a new decade in age … or imagine entering a new epoch, which leads them to behave in ways that suggest an ongoing or failed search for meaning," write the authors of the study, published online this week in the Proceedings of the National Academy of Sciences.

The researchers looked at six surveys and studies related variously to marathon running, infidelity, questioning the meaning in life and suicide. This included 2000 to 2011 data from the U.S. Centers for Disease Control that found the rate of suicide among people in their last year of a decade was higher than rates for any other age.

The researchers also analyzed figures from the website Athlinks, which charts marathon finishing times, and found that runners raced faster when they were 29 or 39 compared to other years, suggesting they trained harder for a race before a milestone birthday of 30 or 40.

They also looked at numbers from the online-dating site Ashley Madison, which helps users have affairs, and found that of the eight-million-plus men on the site, nearly one million of them were precisely aged 29, 39, 49 or 59.

While other birthdays may be substantively more important – turning 16 to drive, 18 to vote and 19 to drink in Canada – these end-of-era birthdays are mostly semantic. So why do they hold such significance? Previous data suggests round numbers are powerful psychological motivators.

"The onset of the new decade causes people to step back and take stock of their lives and assess how meaningful things are," study co-author Hal Hershfield, an assistant professor of marketing at UCLA Anderson School of Management, said from Los Angeles.

Hershfield, who co-authored the paper with Adam Alter, an associate professor of marketing at NYU, compared it to the "fresh start" sentiments of New Year's Eve.

"It's not abnormal to have these big questions and make big decisions like this, but one should consider whether the choices are made for the right reason: Is it just because we're facing this new decade? Or is it because you really want to do whatever it is that you're thinking of doing?" says Hershfield.

He wants to use the research to motivate people to reach for healthier goals during self-reflective end-of-era periods: "Down the line it would be optimal if we could use these age transitions as a motivational push to get people to make the decision to quit smoking, start saving more for retirement, to eat healthier."

Hershfield says the research pricks the notion of a single, heady midlife crisis: "People have folk notions of midlife crises. We shouldn't just look to the midlife for these crises."

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