Deciding which charities to support is a heavy responsibility. Who among us is really qualified to compare the moral worthiness of old people and children, trees and animals, cancer patients and diabetics, Indians and Haitians? Should the question of who is deserving be based on our personal preferences and prejudices alone?

A movement called “effective altruism” is an increasingly popular solution to this quandary. We’re all familiar with utilitarianism—Jeremy Bentham’s notion that we should make decisions that result in the greatest good for the greatest number. Effective altruism, based on the ideas of philosopher Peter Singer, says we should apply a similar logic to philanthropy, allocating our money, time and other resources in a way that will measurably do the most good....