Reaching the end of a decade is quite a milestone in each individual's life. It's the point when a teenager becomes a twenty-something and a fourty-something reaches that peak of their life. But reaching our 20s, 30s, 40s, or 50s can be an overwhelming process, and sometimes those milestone birthdays prompt us to think about who we were the decade before and who we'd like to be the decade ahead.

And at these strange cusps where "nine-enders" (those who have an age ending in a nine) step into the next set of ten, researchers now say that individuals may mark the milestones with a slew of good choices looking towards their future - and maybe just a bad choice or two too.

A new study published in the journal Proceedings of the National Academy of Sciences found that "nine-enders" are far more likely to engage in productive behavior like running a marathon or furthering their educations. But on the flip-side of that very same coin, these individuals are also far more likely to pick up a socially destructive behavior or two, that include extramarital affairs and addictions of a variety of sorts.

"People audit the meaningfulness of their lives as they approach a new decade in chronological age" lead authors of the study, Adam Alter and Hal Hershfield with NYU and UCLA say. And while the midlife crises may be the most well-known examples of how individuals may drastically change their lives at the drop of a hat, when faced with the prospects of their current discontents, the team says that people are prone to these types of shifts even before a midlife crisis, when the end of a decade prompts the analysis of their current situations.

Looking for a way to quantify and directly analyze the search for existential meaning to life, and the drastic ramifications the process may have on a person's behavior, the researchers sought out the effects of changing age in a series of six studies that looked at exercise, extramarital affairs and suide rates amongst individuals ranging in age from 25 to 64. And what the team found from their large-scale questionnaires was that people nearing the end of a decade were far more likely to question whether their lives were meaningful than individuals of any other age.

"Our research suggests that people who are nearing the end of a decade may be more likely to make large purchases (e.g., buy life insurance, invest in retirement savings, pursue cosmetic surgery, etc.)" Alter and Hershfield say. "Being aware of the tendency to do so can help consumers decide if they are making such decisions for the right reasons."

And while milestone birthdays may be a joy to celebrate, the research suggest you may want to take it slow on the drinks for your next party... that is, unless you'd like to get a head start on the bad habits for the next decade.