In an OP-ED column (NY Times) suggesting that persons with power tend to be oblivious to others, Richard Conniff writes:
Researchers led by the psychologist Dacher Keltner took groups of three ordinary volunteers and randomly put one of them in charge. Each trio had a half-hour to work through a boring social survey. Then a researcher came in and left a plateful of precisely five cookies. Care to guess which volunteer typically grabbed an extra cookie? The volunteer who had randomly been assigned the power role was also more likely to eat it with his mouth open, spew crumbs on partners and get cookie detritus on his face and on the table.
Keltner, a social psychologist, says that the behavior of the participants in this study can be understood in terms of approach-inhibition theory which is essentially an economic theory of behavior. According to the theory, people with greater power are often the recipients of greater benefits. They also experience less resistance from other people when they take risks. Accordingly, high-power people take more risks to earn rewards. But Keltner didn't indicate that all experimental participants assigned to the power condition behaved in a manner the theory predicts; they merely did so on average. The theory doesn't explain why manipulating power in a given instance causes some people, but not all people, to become cookie-grabbing slobs.
I'm inclined to view the behavior of Keltner's participants in terms of infantile grandiosity. A position of power can stimulate infantile feelings of omnipotence that are expressed in typically infantile ways that include an outsized sense of entitlement, expansiveness and exhibitionist behavior. Grabbing more cookies, spraying crumbs around and eating with one's mouth opened can be seen in this way.
Perhaps enough individuals react to power with subtle increases in their feelings of omnipotence to explain the statistically significant difference in the behaviors of the Keltner's participants in the power condition versus those in the non-power position. Since the tendency to regress in this manner can vary greatly from one individual to the next, it would make sense that some but not all individuals in the power condition display the regressive behavior of Keltner's cookie monsters.