The idea that we might have a finite source of willpower to tap into was put forward by Roy Baumeister and Dianne Tice. Imagine doing an impossible crossword whilst at the same time resisting a plate of biscuits on a table - you'll give up the crossword sooner than if there weren't any biscuits because you've been using up your willpower. They call this Ego Depletion. However, Slate reports on a new study which throws doubt on the theory:
“Taken at face value, the new Registered Replication Report doesn’t invalidate everything we thought we knew about willpower. A person’s self-control can lapse, of course. We just don’t know exactly when or why. It might even be the case that Baumeister has it exactly right—that people hold a reservoir of mental strength that drains each time we use it. But the two-task method that he and Tice invented 20 years ago now appears to be in doubt. As a result, an entire literature has been rendered suspect.”