The day I entered my 40th lap around the sun – otherwise known as my 39th birthday – was the same day I checked out of my normal life and checked in for a flight halfway across the world. Starting in Melbourne, I embarked on a five-week adventure, travelling through Australia, New Zealand and Japan like some kind of mad, condensed “gap yah”, except grubby hostels were very much traded in for nice hotels with spas.

When I returned to the UK, I had another month off work: four beautiful, indulgent weeks of brunches and saunas and yoga classes and drinking frozen margaritas in the middle of the day while slowly chipping away at my much-talked-about-but-never-before-started YA novel. It was all a bittersweet taste of what life could’ve been like, had I only been smart enough to marry rich.

Before anyone accuses me of falling prey to a minor midlife crisis, I invite you to take a look at your own life. More specifically, mull on what you did in the 12-month periods preceding your big birthdays. You know the ones: those terrifying numbers with zeros at the end, wheeled out every 10 years to scare the bejesus out of us.

The years when we turn something-nine – 29, 39, 49, 59 etc – might, on the face of it, seem like any other year. And yet evidence suggests that age isn’t just a number, after all. Dubbed “nine-enders”, these are often the points at which we make big decisions or changes, blow our lives up, tick off bucket-list activities and generally Get S*** Done.

The “nine-ender” theory was most famously posited by Adam Alter and Hal Hershfield, a marketing professor and a professor of behavioural decision-making and psychology respectively, more than a decade ago. In their 2014 paper, “People search for meaning when they approach a new decade in chronological age”, the pair sought proof to support their hypothesis: looking down the barrel of a significant birthday really does make us reflect on our lives, geeing us up and spurring us on.

As it turned out, evidence wasn’t hard to come by. When they analysed responses from more than 42,000 adults across 100 countries who completed the World Values Survey, they discovered that people whose age ended in nine were more likely than respondents whose ages ended in any other digit to question the meaning or purpose of life.

That quest for meaning seemed to lead, in turn, to tangible impacts. Analysing other data sets, Alter and Hershfield found that nine-ender males were more likely than any other age demographic to have registered for an extramarital dating site, for example. In other words, those approaching a new decade were more motivated to seek out an affair.

There were other compelling examples: nine-enders of any gender had a higher suicide rate than non-nine-enders. On a more positive note, this cohort was also significantly overrepresented when looking at first-time marathon runners. And a separate analysis revealed that marathon runners tended to complete the course faster in their nine-ender years than they had in the two years previously or in the two years afterwards. For good or ill, those years ending in nine seemed to act as a uniquely powerful driver of human behaviour.

Others have tested the theory in the intervening years. A 2019 paper in the Journal of Personality and Social Psychology replicated the results, and also found that imagining entering a new decade caused people to specifically look back, reflect and take stock of their lives. Another landmark work on the “fresh start effect” showed that birthdays, the start of a new week or year, or even the start of a new term, all increase aspirational behaviour. “That literature has replicated across many contexts and now informs real-world interventions, including retirement savings nudges,” says Adam Alter. “The underlying psychology is well-supported.”

I sense instinctively that the next 10 months are going to see seismic shifts that keep rippling outwards

Reflect for a moment, and you might be surprised by just how much the idea rings true. When I mention it to friends and colleagues, I’m amazed by how many of them have their own real-world examples of big shifts that happened on nine-enders. Career changes; relationship milestones; pregnancies; marital breakdowns; relocations or big house moves. Even the quieter, yet arguably just as impactful, alterations seem to coincide with those years. Two different people shared that 29 was the age at which they finally took up the hobbies they’d always dreamed of doing – improv comedy and pole dancing, in case you’re wondering.

My last nine-ender, 29, saw me move in with my then-boyfriend, also 29 – the first time either of us had ever lived with a romantic partner – and get a new job after four years at the same company. My new nine-ender has already started with a bang – that two-month sabbatical, including a trip to a whole other hemisphere, is the only career break I’ve taken since graduating 18 years ago – but it’s also just the beginning. I sense instinctively that the next 10 months are going to see seismic shifts that keep rippling outwards, like endless concentric circles after a pebble has been dropped in the middle of a lake. After multiple false starts, I’m finally in a relationship with a person I’m happy to unironically refer to as the love of my life; it feels like turning the corner into a year of hope and next steps, a future that’s brimming with potential and possibility.

According to Alter, this tendency to make big decisions or changes during our nine-enders is a mix of conscious and unconscious behaviour. “Some are explicit about it. They say out loud, ’I want to run a marathon before I turn 40’,” he tells me. “Those are the easy cases.

“The more common pattern is partial awareness. People feel a pressure they can’t quite name. They become restless or unusually open to a change they wouldn’t have considered the year before. The age is operating as a deadline in the background.”

This invisible deadline exerts a powerful influence because humans don’t experience time as a smooth continuum, according to Alter – even if that’s how clocks and calendars present it.

“The boundaries we draw; the end of a decade in age, the end of a year, a birthday function as natural review points, the way the end of a chapter in a book makes you stop and absorb what you’ve read,” he explains. “When someone is about to turn 30 or 40 or 50, that round number serves as a forcing function. They look back at the decade they’re closing and ask, sometimes explicitly and sometimes just as a background hum: did I do what I wanted to do? If the answer is yes, the decade closes quietly. If the answer is no, they feel pressure to act before the door shuts.”

People feel a pressure they can’t quite name. They become restless or unusually open to a change they wouldn’t have considered the year before

Adam Alter, marketing professor

Some of that pressure produces constructive responses, like first marathons, career changes or creative projects that have been deferred for years. Some of it produces destructive responses, such as affairs. “The motivation is the same; the form the action takes depends on the person and their circumstances.”

And there is, of course, nothing inherently meaningful about the number 10. It’s arbitrary; “We use it because we have 10 fingers,” says Alter. “If we’d evolved with nine fingers, we might be having midlife crises at 36, not 40.” Nevertheless, those round numbers are potent. They carry an undeniable psychological weight across nearly every aspect of our lives. Baseball players hit a disproportionate number of 300 batting averages and disproportionately few 299s. Prices set just below round numbers sell better than prices just above.

As Alter puts it, “a milestone birthday is the same phenomenon applied to the largest possible canvas: a life.” And, while the number may be arbitrary, the act of evaluating where you’re at and striving for more can be genuinely transformative.

So bring on this nine-ender, I say. As long as I steer clear of Ashley Madison, it should be a banner year.