Mid-life crises aren't the only time people make major, life-altering decisions. It turns out anytime we hit a birthday milestone, like 29 or 39, we make life-altering choices in preparation. But why?

Joanna Rothkopf of Salon has found new research published in the Proceedings of the National Academy of Sciences that has the answer: We realize our own mortality. Around the time when we're a year away from these decade milestones, Hal Hershfield, a UCLA psychologist and co-authored of the study, found we become more aware of our age in the grander scheme of things.

“... transitions, they make us step back, they make us evaluate things. Once you’re in the midst of something, it’s easier to keep your head down and just go, go, go. But when you take a step back, that’s when you say, 'Wait, let me see how everything’s going here.'”

These age milestones command meaningful reflection in most people, the study writes:

“...which is linked to a rise in behaviors that suggest a search for or crisis of meaning.”

Around ages 29, 39, and 49, researchers found an uptick in people running marathons for the first time (or training harder to complete one), committing suicide, or having an affair. There's a trend of either unhappiness or intense motivation among these age groups that's hard to ignore. The disruptive habits, perhaps, more so than a positive goal to run more. 

In future research, Hershfield said to Science Us, he'd like to find out what bring some people to seek out affairs or take their own lives. There's the underlying knowledge of taking one's own mortality seriously, but why is there such a divergence in people pursuing positive, healthy goals while other choose to disrupt or end their lives.

Read more at Salon

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