That was my first thought, but it appears they accounted for that — “The researchers designed a series of four experiments to test and manipulate Stanford students’ beliefs about willpower. After a tiring task, those who believed or were led to believe that willpower is a limited resource performed worse on standard concentration tests than those who thought of willpower as something they had more control over.”
Claims that the extent to which will power is exhaustible depends on one’s belief about it’s exhaustibility: http://www.sciencedaily.com/releases/2010/10/101014144318.htm
This needs to be checked for the direction of causality—maybe people have accurate beliefs about how depletable their willpower is.
That was my first thought, but it appears they accounted for that — “The researchers designed a series of four experiments to test and manipulate Stanford students’ beliefs about willpower. After a tiring task, those who believed or were led to believe that willpower is a limited resource performed worse on standard concentration tests than those who thought of willpower as something they had more control over.”