http://www.boston.com/bostonglobe/ideas/articles/2010/07/18/beyond_good_and_evil?mode=PF
GRAY: If you think of yourself as a good-doer, you come to possess increased agency and decreased experience. Same with an evil-doer. So what we have people do is randomly assign them to either do something good for others or receive something themselves, and then measure their physical endurance by holding a weight. And what we find is that those who are given the opportunity to help others actually become physically stronger, possess more endurance....And what we find is that those who are given the chance to do evil increase in agency in kind, but also a little bit more than those who do good.
IDEAS: Why a little bit more, do you think?
GRAY: I think it’s because for the average person with a conscience, it’s a little harder to do evil....I really need to overcome my qualms to do it, and once I do it, I feel like I must be even more powerful.

http://www.scientificamerican.com/article.cfm?id=the-willpower-paradox&print=true
It is the difference between “Will I do this?” and “I will do this.”
The results were provocative. People with wondering minds completed significantly more anagrams than did those with willful minds. In other words, the people who kept their minds open were more goal-directed and more motivated than those who declared their objective to themselves.
These findings are counterintuitive. Think about it. Why would asserting one’s intentions undermine rather than advance a stated goal? Perhaps, Senay hypothesized, it is because questions by their nature speak to possibility and freedom of choice.
...It indicates that those with questioning minds were more intrinsically motivated to change. They were looking for a positive inspiration from within, rather than attempting to hold themselves to a rigid standard.
...those who were asserting their willpower were in effect closing their minds and narrowing their view of their future.