Economists believe that people are fundamentally motivated by personal gain. Which certainly seems to be true of the people around them:

Does what we believe about human motivation matter? In an experimental study of private contributions to a common project, two sociologists from the University of Wisconsin, Gerald Marwell and Ruth Ames, found that first-year graduate students in economics contributed an average of less than half the amount contributed by students from other disciplines.

Other studies have found that repeated exposure to the self-interest model makes selfish behavior more likely. In one experiment, for example, the cooperation rates of economics majors fell short of those of nonmajors, and the difference grew the longer the students had been in their respective majors.

But correlation is not causation. It's also possible, for instance, that economics majors tend to be greedheads (and to think everyone else is a greedhead as well) because it's greedheads who are disproportionately drawn to economics...