Alter, A.* & Hershfield, H.E.* (2014). Proceedings of the National Academy of Sciences, 111, 17066-17070.
Although humans measure time using a continuous scale, certain numerical ages inspire greater self-reflection than others. Six studies show that adults undertake a search for existential meaning when they approach a new decade in age (e.g., at ages 29, 39, 49, etc.) or imagine entering a new epoch, which leads them to behave in ways that suggest an ongoing or failed search for meaning (e.g., by exercising more vigorously, seeking extramarital affairs, or choosing to end their lives).