Most of our thinking about how to influence human behavior — how to get people to pay taxes, to obey laws, to not steal from each other — rests on the model of homo economicus. This creature, first sketched by economists more than a century ago, is generally out for his own rational self-interest. Most of our laws today are designed to influence behavior through carrots and sticks that appeal to selfishness. We motivate CEOs through material incentive in pay-for-performance schemes. ...If we want to appeal to conscience instead of greed — a potentially much cheaper strategy — we first have to recognize how common and powerful pro-social behavior already is.