I’ve never written a New Year’s resolution list, but every December, as the end of the year approaches, I find myself reflecting on the past twelve months’ accomplishments and failures, and planning how to do better (or stay the course if everything is happy) in the New Year. My very vague master plan for 2015 includes getting back on a regular exercise schedule and unpacking the left-over boxes from my move from Phoenix to Brooklyn last summer. Past years have included more ambitious plans, such as getting a new job, or writing a book, or having a child.

Arbitrary periods of increased self-examination and evaluation are not restricted to the end of the calendar year, but they appear to resurface, with great intensity, whenever we approach a new decade in life. Recent research published in PNAS indicates that people tend to search for existential meaning at 29, 39, 49, etc., years of age, as well as when they imagine entering a new decade. This existential quest can have powerful consequences, both adaptive and maladaptive, that manifest in more vigorous exercise regimes, but also in increased numbers of extramarital affairs and suicide attempts, at the end of each decade.

Although we measure time in a continuous scale, some points in our lives appear to be vested with special significance. Graduation, marriage, becoming a parent, are obvious milestones, but because many societies divide our life span in decades, we also assign particular import to, and measure our aging in, discrete 10-year intervals (“the twenties”, “the thirties”, et cetera).

The researchers reasoned that when people examine their lives, they find them either meaningful, or lacking in meaning in one or more important fronts (such as having purpose, values that distinguish right from wrong, meaningful social interactions, self-worth). Those who find their lives meaningful will keep doing what they’re doing, but those who find them lacking may react adaptively (by implementing behaviors that are likely to generate meaning, such as running a marathon) or maladaptively (by adopting behaviors detrimental to, or incompatible with, finding existential meaning, such as committing suicide).

The series of studies found that “9-enders” thought more about – and questioned more-- the meaning and purpose of life than non-9-enders. This increased search for purpose applied not only to people at the end of chronological decades, but it also happened to people of all ages when they imagined entering a new decade.

Next, the scientists calculated the frequency of registrations of male 9-enders between the ages of 25 and 64, relative to other males, on a dating website dedicated to people seeking extramarital affairs. Registrations of 9-enders were 17.88% more prevalent than expected by chance, and most frequent at 49 years of age. Now, that’s experimental evidence in support of the midlife crisis concept (sadly, the paper provides no data on the frequency of 9-enders among red sports cars buyers).

Suicide rates throughout the US were also slightly higher for 9-enders than among those with ages ending in any other digit.

Finally, the researchers analyzed the participation of 9-enders and non-9-enders of both genders in marathons. This study addressed the potential concern that people were lying about their age in the dating website (placing themselves at the end of the previous decade, rather than in their actual decade). If people thought that taking a few years off might give them an edge in the dating game, it might have inflated the number of 9-ender registrations. In contrast, athletic events divide participation into 5-year brackets (for instance, 35 to 39 years), so dishonest 9-enders might round their age up to the next decade, which would give them an advantage as the youngest members of that bracket.

Male and female 9-enders were faster than runners of ages in the two previous and subsequent years, suggesting a more rigorous training regime at the end of the decade than at other ages. Also, 9-enders were overrepresented among first-time marathon runners. Of 500 first-time runners randomly drawn from the Athlinks website, 74 of them were 9-enders, which amounts to a 9-ender overrepresentation of 48%.

The scientists concluded that even though we do age continuously, we ponder more the passage of time at some arbitrary times than others, which can prompt major --and sometimes irreparable-- courses of action in our lives.

How can we use these imagined milestones to our benefit? One inciting finding from this research is that we can trigger increased self-evaluation by the act of imagining how we will feel when the decade ends. Say you’re smack in the middle of your decade, but seeking to improve your running times sooner rather than later: you could psych yourself up by pondering the start of the next decade, which is, after all, not that far away. Time flies!

For those 9-enders among you… maybe put off purchasing that sports car (or cheating on your spouse!) for one short year, and see how you feel about it then.

Wherever you are in your lifespan… Here’s to new, arbitrary beginnings. I wish you a joyful, meaningful, 2015.

Happy New Year!