There’s something different about the number nine. We know this intuitively, right? It’s why everything on late-night infomercials is priced at $999.99 instead of a cool thou, and why a dress marked up to $39 sells better than the same dress sells at $34. It’s like there’s an urgency associated with being pushed right up against a big round number — something that makes us want to act before the ticker rolls over to zero.

Maybe I’m thinking about this because I’m facing a big birthday this year — the one that used to mean someone would buy you a mug with “Over the Hill” on it — or maybe because it’s 2019, an uneasy, uneven year that stretches out long and desolate, standing between us and the glittering promise of 2020.

Turns out there’s actually data that suggests being an age that ends in nine—29, 39, 49, 59, etc. — may lead us to do things we are less likely to do at other ages.

I first came across the concept of the “nine-ender” in Daniel Pink’s recent book When: The Scientific Secrets of Perfect Timing. The term was coined by marketing psychologists Adam Alter and Hal Hershfield, who in 2015 published a paper arguing that people who are staring down the end of a decade engage in “meaning-seeking behaviors” — both positive and self-destructive — more frequently than they do at other ages.

It started with greeting cards. The researchers wondered: Why is the supermarket greeting-card rack dominated by cards commemorating big milestone birthdays?

“It’s funny because it’s arbitrary,” says Hershfield, now a professor of marketing at UCLA’s Anderson School of Management. “Turning 30 or 40 or 50 doesn’t really mean anything; it’s not like it’s qualitatively different.”

“That milestone age is an opportunity to either do really stupid shit or get a grip, reevaluate your life, and do something about it.”

The researchers wanted to know if that arbitrary emphasis on milestone birthdays might lead people to change something about their psychology or behavior as they approached a new decade. To find out, they looked at a number of data sets, including marathon sign-ups and race times, suicide rates, and profile data from Ashley Madison (the “have an affair” website). They found that nine-enders were overrepresented in all those areas, sometimes by a lot: As Pink explains in his book, 29-year-olds were about twice as likely to run a marathon as 28- or 30-year-olds, and 49-year-olds were three times more likely to do so than people just one year older. Also, runners who had done multiple marathons posted faster finish times at 29 and 39 than in the two years before or after hitting their nine-ender ages.

While reaching the end of a decade might inspire some to turn over a healthy new leaf, the study found that being a nine-ender also correlates with more self-destructive acts. Suicide rates among nine-enders were higher than among people whose ages ended in any other digit, and nine-enders showed up on Ashley Madison about 18 percent more frequently than chance would predict.

“These milestone birthdays end up disproportionately affecting us, making us take stock of our lives” Hershfield says. “When we do that, we’re more vulnerable to seeking out meaning in a variety of ways.”

The extent to which this behavior is conscious is up for debate. When Alter and Hershfield analyzed the responses of 42,063 adults across 100 countries who filled out the World Values Survey, they found that “nine-enders reported questioning the meaning or purpose of life more than respondents whose ages ended in any other digit,” suggesting some degree of self-awareness.

“That milestone age is an opportunity to either do really stupid shit or get a grip, reevaluate your life, and do something about it,” echoes Talya Miron-Shatz, a former Princeton researcher whose 2015 paper supported the idea that big birthdays cause people to take stock. When evaluating how satisfied they are with their lives, that study found, people at milestone ages tend to take the long view, placing more emphasis on their health and less on merely feeling positive emotions from day to day.

Subsequent research backed up some of Alter and Hershfield’s findings. The website IllicitEncounters.com (Ashley Madison with a British accent) also claims people are most likely to cheat when they hit age 39, followed by 49, then 29.

But not everyone is convinced, and not just because research from a site called IllicitEncounters.com is probably not rigorously peer reviewed. What about the seemingly obvious notion that cheaters might not have qualms about lying about their age on a dating site? It’s not hard to imagine that a graying 56-year-old might try to hook a younger mistress by rounding his age down to, say, 49.

A team of German researchers picked up on this as one of the possible flaws in Alter and Hershfield’s nine-enders study. In an attempt to replicate those findings, they interrogated data from a 30-year longitudinal study of tens of thousands of German households to see if individuals reported changes in things like BMI, marital status, and life satisfaction as they neared the end of a decade — signs that might indicate the search for meaning hypothesized by Alter and Hershfield.

“We found no evidence for a change in attitudes or behavior that would indicate an intensified search for meaning in life when adults approach a new decade in chronological age,” the Germans retorted curtly, in what I imagine to be the voice of Christoph Waltz.

“We believe none of these challenges is compelling,” Hershfield and Alter shot back, with classic American bravado. They stand by their study but admit that more research is needed.

Regardless of how solid the data turns out to be, it sure seems to resonate. While researching this story, I heard from people who made life-changing moves at nine-ender ages, including getting married (at 29), getting divorced (at 39), getting remarried (at 79), becoming a surrogate, applying to law school, and even learning how to ride a bike (all at 39).

If we as individuals search for meaning at the end of a decade, maybe the same holds true for humanity as a unit.

Hershfield, whose work focuses on the psychology of financial decision-making, has not continued studying the nine-ender phenomenon, but he’s curious to know if it shows up around other decisions, like taking out a life insurance policy or, more darkly, committing a mass shooting — “Anything where you could say it’s a possible risk factor for some negative behavior that represents a failed search for meaning,” he says.

“The interesting next question is: What are the precursors for someone going down a more positive path versus a more negative path when facing milestone birthdays?” Hershfield says. “It would be worth knowing if these are good times to nudge consumers toward making decisions that may benefit them in the future, like making a higher contribution to their retirement account or signing up for a will.”

I wondered if this nine-ender psychology could ever translate on a bigger scale, like world politics. If we as individuals search for meaning at the end of a decade, maybe the same holds true for humanity as a unit — what Carl Jung called the collective unconscious. Would entering a nine-ender year affect how we as a global community make decisions about the future? Could 2019 be the year we finally take stock of the world and decide to make real strides on, say, climate change?

Hershfield doesn’t buy it. “The milestone aspect is a convenient shortcut to set for deadlines for things like sustainability, but I’m not sure turning from 2019 to 2020 is enough of a psychological event to make people take stock and engage in meaning-making behavior,” he says, casually smashing all of my hopes for a better 2019. Like me, Hershfield happens to be a nine-ender himself at the moment. We’ll both turn 40 this summer. Neither of us has signed up for a marathon. Yet.