Peer group influence
According to a new study by Michelle van Dellen, a psychologist at the University of Georgia, self-control contains a large social component; the ability to resist temptation is contagious.
The paper consists of five clever studies, each of which demonstrates the influence of our peer group on our self-control decisions. For instance, in one study 71 undergraduates watched a stranger exert self-control by choosing a carrot instead of a cookie, while others watched people eat the cookie instead of the carrot.
That's all that happened: the volunteers had no other interaction with the eaters. Nevertheless, the performance of the subjects was significantly altered on a subsequent test of self-control. People who watched the carrot-eaters had more discipline than those who watched the cookie-eaters.