I've been reading the fascinating work of Paul Slovic, a psychologist who runs the social-science think tank Decision Research. He studies a troubling paradox in human empathy: We'll usually race to help a single stranger in dire straits, while ignoring huge numbers of people in precisely the same plight.

We're very good at processing the plight of tiny groups of people but horrible at conceptualizing the suffering of large ones.

people were asked to donate money to help a dying child. When a second set of subjects was asked to donate to a group of eight children dying of the same cause, the average donation was 50 percent lower.

Slovic suspects this stuff is hardwired. Psychologists have long observed that our ability to discriminate among quantities is finely tuned when dealing with small amounts but quickly degrades as the numbers get larger.

We'll break the bank to save Baby Jessica, but when half of Africa is dying, we're buying iPhones.