When the Starbursts cost a cent apiece, the average number of candies per customer was 3.5, but when the price went down to zero, the average went down to 1.1 per customer. The students limited themselves to a large degree when the candy was free. In fact, almost all the students applied a very simple social-norm rule in this situation—they politely took one and only one Starburst. ... What these results mean is that when price is not a part of the exchange, we become less selfish maximizers and start caring more about the welfare of others. We saw this demonstrated by the fact that when the price decreased to zero, customers restrained themselves and took far fewer units.
Dan Ariely, Predictably Irrational: The Hidden Forces That Shape Our Decisions
free, politeness, price