Why People Whose Age Ends in Nine Are More Likely to Run Their First Marathon
People whose age ends in nine are 48 percent more likely than everyone else to be running their first marathon. This intriguing and specific prediction comes not from astrology but from rigorous psychological research. It reflects a broader pattern observed by psychologists Adam Alter and Hal Hershfield in 2014, shedding new light on why some individuals quietly diminish the importance of their birthdays. This phenomenon is distinct from the common narrative about dreading aging.
Six Studies, One Consistent Pattern
In their landmark study published in the Proceedings of the National Academy of Sciences, Adam Alter of New York University and Hal Hershfield of UCLA explored how people psychologically experience age differently. Their research centered on the idea that chronological time is not felt evenly throughout life. Specifically, the years ending in nine—ages 29, 39, 49, and so forth, which they termed “9-enders”—carry disproportionate psychological weight.
Leveraging the World Values Survey, which includes data from 42,063 adults across 100 countries, they found that 9-enders reported reflecting on the meaning and purpose of their lives more frequently than people at other ages. While this effect was modest alone, it consistently appeared across multiple contexts. For example, in a random sample of 500 first-time marathon runners across five major marathons, 74 were 9-enders—an overrepresentation of 48 percent. Similarly, among over eight million men registered on a website designed for extramarital affairs, men who were 9-enders were nearly 18 percent more likely to be active. Furthermore, U.S. Centers for Disease Control and Prevention data showed a slight uptick in suicide rates among 9-enders compared to other age groups.
Alter and Hershfield interpret these outcomes on a spectrum: training for a first marathon represents an adaptive, constructive response—a way to restore purpose and capability. In contrast, engaging in an affair or attempting suicide may reflect attempts to escape or end life perceived as disappointing. Thus, the same psychological prompt—a waning sense of meaning as a decade closes—can lead to very different behaviors depending on individual circumstances.
Why Turning 29 Feels Different from Turning 30
The researchers propose that birthdays just before a new decade act as temporal landmarks—psychological milestones that encourage people to evaluate their lives against their prior expectations. This concept aligns with the “fresh start effect” identified by Hengchen Dai, Katherine Milkman, and Jason Riis, who found that clear temporal boundaries (such as a new year, month, or birthday) motivate aspirational behaviors by psychologically separating people from their less successful pasts.
What makes 9-ender birthdays particularly impactful is not the age itself, but their proximity to a culturally significant milestone. Turning 29 sits directly before the symbolic “big” birthday of 30, creating an especially potent moment for self-reflection and motivation.
The Debate Over the Evidence
Despite the compelling correlations reported by Alter and Hershfield, the findings are not universally accepted. In 2015, Simon Kühne, Thorsten Schneider, and David Richter published a response using German panel data that tracked the same individuals over time rather than comparing groups of different ages. Their analysis found no evidence for the kind of crisis-like changes described in the original study. Alter and Hershfield responded by defending their methodology and data, but the scientific community has yet to reach consensus.
It is important to emphasize that these findings are correlational and should not be interpreted as deterministic or diagnostic. If someone experiences thoughts of suicide or severe distress related to their birthday or any other time, it is crucial to seek immediate help from medical professionals, crisis lines, or trained counselors rather than relying on statistical studies.
What We Can Take Away
Regardless of ongoing debate about the exact size or nature of the effect, the theory offers a useful perspective on the psychology of birthdays. If turning 29 or 39 naturally triggers a semi-automatic, involuntary bout of self-assessment, then a birthday celebration at that time may feel less like a joyful event and more like an unwelcome spotlight on a personal reckoning. Choosing to downplay such a birthday may be less about denying age and more about avoiding an audience for an internal audit that was never consciously invited, but rather imposed by the calendar.
Understanding this subtle emotional landscape can foster greater empathy for those who approach milestone birthdays with ambivalence or quiet reflection, rather than overt celebration.
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