People tend to get richer as they age. Five dollars simply means more to you when you’re 9 than when you’re 49. Also, the future is uncertain. We can’t always trust there’ll be one. It’s likely some kids in Walter Mischel’s famous “marshmallow experiment”—which asked kids to wait to eat a marshmallow to get another one—didn’t actually believe that the experimenter would come through with the second marshmallow, and so ate the first marshmallow right away. Saving for retirement makes no sense if in five years a massive meteor cuts human civilization short. Economists call this the “catastrophe” or “hazard” rate.