It can get terribly expensive for seniors — people in their 60s — to keep working instead of retiring, even if they do so because they fear running out of money when they do retire.

A new study from Alan J. Auerbach, Laurence J. Kotlikoff, Darryl Koehler and Manni Yu found that those working seniors can end up forking over their hard-earned money at a much higher rate than do millionaires and billionaires.

How is such a thing possible? There are actually penalties for working that stem from approximately 30 major federal and state tax and transfer programs. The project, funded by the Sloan Foundation, analyzed those penalties, as well as considering how the additional earnings, a part of which will generally be saved, will affect future taxes and transfers. The entire impact is summarized in what is called the worker's "lifetime marginal net tax rate."