Imagine you are setting out on a dangerous expedition through the Arctic on a limited budget. The grizzled old prospector at the general store shakes his head sadly: you can't afford everything you need; you'll just have to purchase the bare essentials and hope you get lucky. But what is essential? Should you buy the warmest parka, if it means you can't afford a sleeping bag? Should you bring an extra week's food, just in case, even if it means going without a rifle? Or can you buy the rifle, leave the food, and hunt for your dinner?

And how about the field guide to Arctic flowers? You like flowers, and you'd hate to feel like you're failing to appreciate the harsh yet delicate environment around you. And a digital camera, of course - if you make it back alive, you'll have to put the Arctic expedition pics up on Facebook. And a hand-crafted scarf with authentic Inuit tribal patterns woven from organic fibres! Wicked!

...but of course buying any of those items would be insane. The problem is what economists call opportunity costs: buying one thing costs money that could be used to buy others. A hand-crafted designer scarf might have some value in the Arctic, but it would cost so much it would prevent you from buying much more important things. And when your life is on the line, things like impressing your friends and buying organic pale in comparison. You have one goal - staying alive - and your only problem is how to distribute your resources to keep your chances as high as possible. These sorts of economics concepts are natural enough when faced with a journey through the freezing tundra.

But they are decidedly not natural when facing a decision about charitable giving. Most donors say they want to "help people". If that's true, they should try to distribute their resources to help people as much as possible. Most people don't. In the "Buy A Brushstroke" campaign, eleven thousand British donors gave a total of 550,000 pounds to keep the famous painting "Blue Rigi" in a UK museum. If they had given that 550,000 pounds to buy better sanitation systems in African villages instead, the latest statistics suggest it would have saved the lives of about one thousand two hundred people from disease. Each individual $50 donation could have given a year of normal life back to a Third Worlder afflicted with a disabling condition like blindness or limb deformity..