According to neoclassical economics, rational beings do whatever they need to in order to maximize their own wealth. However, when people make sacrifices to benefit others without expecting a personal reward, they are thought to behave altruistically (Rushton, 1984). Common applications of this pro-social behavior include volunteering, philanthropy, and helping others in emergencies (Piliavin & Charng, 1990).
Altruism is evident in a number of research findings, such as dictator games. In this game, one participant proposes how to split a reward between himself and another random participant. While some proposers (dictators) keep the entire reward for themselves, many will also voluntarily share some portion of the reward (Fehr & Schmidt, 1999).
Fehr, E., & Schmidt, K. M. (1999). A theory of fairness, competition, and cooperation. The Quarterly Journal of Economics, 114, 817-868.
Piliavin, J. A., & Charng, H. W. (1990). Altruism: A review of recent theory and research. Annual Review of Sociology, 16(1), 27-65.
Rushton, J. P. (1984). The altruistic personality. In Development and maintenance of prosocial behavior (pp. 271-290). Boston, MA: Springer.