And as Prof. Angus Deaton of Princeton University has pointed out, many of the problems of poverty boil down to bad politics, and we don’t know how to use philanthropy to fix that. If corruption drains away donated funds, for example, charity could even be counterproductive by propping up bad governments.

Sometimes we simply can’t know in advance how important a donation will turn out to be. For example, the financier John A. Paulson’s recently announced $400 million gift to Harvard may be questioned on the grounds that Harvard already has more money than any university in the world, and surely is not in dire need of more. But do we really know that providing extra support for engineering and applied sciences at Harvard — the purpose of the donation — will not turn into globally worthwhile projects? Innovations from Harvard may end up helping developing economies substantially. And even if most of Mr. Paulson’s donation isn’t spent soon, the money is being invested in ways that could create jobs and bolster productivity.

In addition, donor motivation may place limits on the applicability of the effective-altruism precepts. Given that a lot of donors are driven by emotion, pushing them to be more reasonable might backfire. Excessively cerebral donors might respond with so much self-restraint that they end up giving less to charity. If they are no longer driven by emotion, they may earn and save less in the first place.

Still, those objections aside, it is hard to believe that Americans cannot improve their practice of philanthropy. In a world of high income inequality, we will have a much better future if donations from the very rich can bring some of the social benefits associated with the past generosity of Mellon, Carnegie, Rockefeller and others. And donations from less wealthy people can help a great deal if they are well directed.

Early in his book, Professor MacAskill considers the provocative hypothesis that the best man ever to have lived was a Ukrainian named Viktor Zhdanov. Working with the World Health Organization, Mr. Zhdanov called for a systematic campaign to eradicate smallpox. He presented a visionary plan, sold the organization on the idea and, by accelerating the end of smallpox, probably saved many millions of lives. If you haven’t heard of him, or haven’t thought of him lately, that’s evidence that the effective-altruism movement has something to offer. Even for small givers, a more rational approach to philanthropy can focus attention on areas that make the biggest enduring contribution to human welfare.