Suppose you volunteer to participate in a psychological experiment. You answer a set of questions and receive a small cash payment in return. As you are leaving the lab, you are handed an envelope from Save the Children and a photo of a starving seven-year-old girl, Rokia, and asked if you would like to donate some or all of your earnings to help her. Another group of volunteers is invited to contribute to help a famished seven-year-old boy, Moussa, while a third group is shown the photos of both Rokia and Moussa and asked to contribute to help both of the children.

Now, here’s the rub: two lives should in theory draw more support than one, but in this experiment, people gave more to the individual children than they did when Rokia and Moussa were paired.

New research from psychologist Paul Slovic and his colleagues shows that what’s called “psychic numbing” mounts as the number of suffering people grows. “The identified individual victim, with a face and a name, has no peer,” they write. That’s not exactly news. People ranging from Mother Teresa (“If I look at the mass, I will never act. If I look at the one, I will”) to, reportedly, Stalin (“A single death is a tragedy; a million deaths is a statistic”) have long recognized that people confronted by large-scale human suffering are often overwhelmed. But it is a new and unsettling twist that compassion begins to fail with the mere addition of a second person.

Slovic is a professor of psychology at the University of Oregon and the president of Decision Research, a nonprofit he founded in 1976. This new study extends a line of work on psychic numbing that he has been involved in for a decade. Slovic suggests that psychic...

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