The saving behavior varies markedly across individuals, in particular with respect to retirement. While some save very little, other save a lot, and this variation goes much beyond what can be explained by variations in income. In a pair of papers, Mariacristina De Nardi, Eric French and John Bailey Jones look at various explanations for this variation.
In the first one, they document that a very important factor in the savings for retirement decision of people is the expected (out-of-pocket) expenses in health care. This explains in particular why asset rich retirees run down their wealth so slowly in the United States, even with Medicare.
In the second one, they show that life expectancy is a major determinant of savings behavior. People have usually a rather good idea of their life expectancy, and they behave accordingly. What they exploit here is that there are some measurable characteristics that are known to influence, or at least to be correlated with, life expectancy, such as health, gender and permanent income. Basically, people are afraid of outliving their retirement assets and save accordingly. Viewed this way, those who save little do it rationally, knowing that they will not live long. And they should not be criticized by those who try to impose on them cookie-cutter retirement plans, like "you should retire with a million dollars in assets."