You are thinking of donating to a worthy cause. Good. But to which cause should you give?

If you seek help from professional philanthropy advisers, the chances are that they won’t have much to say about this vital question. They will guide you, to be sure, through an array of charitable options. But the prevailing assumption in their field is that we shouldn’t, or perhaps can’t, make objective judgments about which options are better than others.

Take Rockefeller Philanthropy Advisors , one of the world’s largest philanthropic service organizations. Its Web site offers a downloadable pamphlet with a chart showing areas to which a philanthropist might give: health and safety; education; arts, culture and heritage; human and civil rights; economic security; and environment. The Web site then asks, “What is the most urgent issue?” and answers by saying, “There’s obviously no objective answer to that question.”

Is this true? I don’t think so. Compare, for instance, two of the Rockefeller Philanthropy Advisors’ categories: “health and safety” and “arts, culture and heritage.” To me it seems clear that there are objective reasons for thinking we may be able to do more good in one of these areas than in another.

Suppose your local art museum is seeking funds to build a new wing to better display its collection. The museum asks you for a donation for that purpose. Let’s say that you could afford to give $100,000. At the same time, you are asked to donate to an organization seeking to reduce the incidence of trachoma, an eye disease caused by an infectious micro-organism that affects children in developing countries. Trachoma causes people to slowly lose their sight, typically culminating in their becoming blind between 30 and 40 years of age. It is preventable. You do some research and learn that each $100 you donate could prevent a person’s experiencing 15 years of impaired vision followed by another 15 years of blindness. So for $100,000 you could prevent 1,000 people from losing their sight.

Given this choice, where would $100,000 do the most good? Which expenditure is likely to lead to the bigger improvement in the lives of those affected by it?

On one side we have 1,000 people spared 15 years of impaired vision followed by 15 years of blindness, with all the ensuing problems that that would cause for poor people with no social security. What do we have on the other side?

Suppose the new museum wing will cost $50 million, and over the 50 years of its expected usefulness, one million people will enjoy seeing it each year, for a total of 50 million enhanced museum visits. Since you would contribute 1/500th of the cost, you could claim credit for the enhanced aesthetic experiences of 100,000 visitors. How does that compare with saving 1,000 people from 15 years of blindness?

To answer, try a thought experiment. Suppose you have a choice between visiting the art museum, including its new wing, or going to see the museum without visiting the new wing. Naturally, you would prefer to see it with the new wing. But now imagine that an evil demon declares that out of every 100 people who see the new wing, he will choose one, at random, and inflict 15 years of blindness on that person. Would you still visit the new wing? You’d have to be nuts. Even if the evil demon blinded only one person in every 1,000, in my judgment, and I bet in yours, seeing the new wing still would not be worth the risk.

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.