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The Science of Milestone Birthdays

Take advantage of the "fresh-start effect" as you flip that calendar page.

Key points

  • People tend to pause and take stock of their lives when they find themselves approaching a milestone birthday.
  • A milestone birthday can be the perfect time to take advantage of the “fresh-start effect.”
  • Milestone birthdays spur mental accounting that motivates movement toward desired goals.

In a couple of months, I’ll be celebrating a milestone birthday. I’ll be exiting one decade of my life and entering another. While I’m not the kind of person who tends to mark milestone birthdays in a big, flashy way, I do pay attention to them, treating each as an opportunity to check in with myself and to map out the next leg of my journey through life.

The science of milestone birthdays

Apparently, I’m not alone when it comes to indulging in a bit of milestone-birthday navel-gazing. Studies show that a lot of people pause to take stock of their lives when they find themselves approaching a milestone birthday; and, according to one group of researchers, the effect is particularly pronounced for those of us whose current age ends in a nine (age 39, 49, or 59, for example). In fact, they even gave us our own name, referring to us as “9-enders”! Apparently, we “9-enders” are likely to engage in a lot of introspection, treating a milestone birthday as an invitation to connect the dots between where we’ve been, where we are right now, and where we hope to head next in our lives.

For this reason, a milestone birthday can be the perfect time to set new goals for yourself. As you head into a fresh decade in your life, you can take advantage of what researchers at The Wharton School at the University of Pennsylvania have described as the “fresh-start effect.” Basically, you’re distinguishing between old you and new you: the person you’ve been and the person you hope to become.

You’re able to make this distinction by performing a bit of mental gymnastics. You simply need to convince your brain that you’re closing the books on the previous decade in your life and entering into a whole new “mental accounting period.” It’s a pretty cool trick!

My own fresh start

When I was in my 40s, I was completely sedentary. I did everything I could to avoid anything even remotely resembling physical activity. But as I started getting closer to my 50th birthday, I recognized that something had to change. I needed to make an effort to move my body more often. And so I decided that the “me” who would soon be entering her 50s was going to be a physically active person. That’s exactly what happened. I made the identity shift and became a person who walks for roughly an hour every day. I’m still that person a decade later!

Making it a birthday worth celebrating

If the fresh-start effect sounds familiar, that’s because it’s the very same phenomenon that fuels all those New Year’s resolutions: the desire to become a new person this year, as opposed to the version of yourself that you happened to be last year. In much the same way, celebrating a milestone birthday can give you the nudge needed to take a step back from the day-to-day and think deeply about what you actually want for yourself moving forward.

I find that really exciting—an invitation to imagine new and different things for ourselves as we grow older.

Happy birthday to us!


Alter, A. L., & Hershfield, H. E. (2014). People search for meaning when they approach a new decade in chronological age. Proceedings of the National Academy of Sciences of the United States of America, 111(48), 17066–17070.

Dai, H., Milkman, K. L., & Riis, J. (2014). The fresh start effect: Temporal landmarks motivate aspirational behavior. Management Science, 60(10), 2563-2582.

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