If you agree, then you are saying, in effect, that the harm of one person’s becoming blind outweighs the benefits received by 1,000 people visiting the new wing. Therefore a donation that saves one person from becoming blind would be better value than a donation that enables 1,000 people to visit the new wing. But your donation to the organization preventing trachoma will save not just one but 10 people from becoming blind for every 1,000 people it could provide with an enhanced museum experience. Hence a donation to prevent trachoma offers at least 10 times the value of giving to the museum.

This method of comparing benefits is used by economists to judge how much people value certain states of affairs. It’s open to criticism because many people appear to have irrational attitudes toward the small risks of very bad things happening. (That’s why we need legislation requiring people to fasten their seat belts.) Still, in many cases, including the one we are now considering, the answer is clear enough.

This is, of course, only one example of how we ought to choose between areas of philanthropy. Some choices are relatively easy and others are much more difficult. In general, where human welfare is concerned, we will achieve more if we help those in extreme poverty in developing countries, as our dollars go much further there. But the choice between, say, helping the global poor directly, and helping them, and all future generations, by trying to reduce greenhouse gas emissions, is more difficult. So, too, is the choice between helping humans and reducing the vast amount of suffering we inflict on nonhuman animals.

But new developments are making these decisions easier. Until recently, it wasn’t even possible to find out which charities were the most effective within their own fields. Serious evaluation of charities helping people in extreme poverty began six years ago with the creation of the nonprofit charity evaluator GiveWell.

Now we can be highly confident that a donation to, for example, the Against Malaria Foundation will save lives and reduce the incidence of malaria, and that giving to the Schistosomiasis Control Initiative will, at very low cost, reduce the incidence of neglected tropical diseases, especially those caused by parasites. More experimental is GiveDirectly, which will transfer at least 90 cents of every dollar you give to an extremely low-income African family. Initial studies show that these donations have long-term benefits for the recipients.

“Effective altruism,” as this evidence-based approach to charity is known, is an emerging international movement. Not content with merely making the world a better place, its adherents want to use their talents and resources to make the biggest possible positive difference to the world. Thinking about which fields offer the most positive impact for your time and money is still in its infancy, but with more effective altruists researching the issues, we are starting to see real progress.