I wondered that too. But they covered that objection in the paper. Study #2. They manipulated people's beliefs about willpower by administering a "push poll", and then tested willpower depletion.
Excellent point.
However, it seems reasonable to me that push polling about someone's future behavior will lead them to act consistently with the signal they just sent in the poll - like in Cialdini's Influence, where people are polled on whether they like to go to opera, or give charitably, by some attractive person they want to impress, and then after affirming are ambushed with a sales pitch (they thought it was an innocent poll but are trapped by their answers). So it seems reasonable to assume that those who were push-polled into believing they will b...
Stanford Report has a university public press release about a recent paper [subscription required] in Psychological Science. The paper is available for free from a website of one of the authors.
The gist is that they find evidence against the (currently fashionable) hypothesis that willpower is an expendable resource. Here is the leader:
(HT: Brashman, as posted on HackerNews.)