Another quotation from the article:
"They also found that leading up to final exam week, students who bought into the limited resource theory ate junk food 24 percent more often than those who believed they had more control in resisting temptation. The limited resource believers also procrastinated 35 percent more than the other group."
Perhaps people with poor self-control, who would eat junk food and procrastinate anyway, are more prone to believe that willpower is biologically limited. This saves them from having to take responsibility for their negative behaviour.
If that were the case, the causality would run in the opposite direction.
In study #2, they manipulated people's beliefs about willpower by administering a "push poll", and then tested willpower depletion.
How did they determine how well the push poll influenced that belief? How did they ensure it didn't work better on those who already had less willpower?
They just randomly assign people to group A or B. Group A is told about the actual theory that willpower is a finite resource (“Working on a strenuous mental task can make you feel tired such that you need a break before accomplishing a new task”). Group B is told a competing theory that willpower compounds (“Sometimes, working on a strenuous mental task can make you feel energized for further challenging activities”). They administered surveys to confirm that they had successfully influence each groups opinions.
Then, the experimenters tested the degree to which using willpower became harder or easier throughout a task. On average, group A exhibited willpower fatigue but group B did not.
This really bothered me. Maybe if you think that willpower is not easy depleted, it is because you have more of it than the people who think it is.
This being said, I personnaly believe that if willpower is limited, it can at least be trained to seem like it isn't.