Stalin famously said that “A single death is a tragedy. A million deaths is a statistic.” Sadly, it turns out that Stalin’s observation is psychologically accurate. That, at least, is the conclusion of Paul Slovic, a scientist at the University of Oregon. Slovic set out to answer a tragically simple question: Why do good people ignore mass murder and genocide?

The answer may lie in human psychology. Specifically, it is our inability to comprehend numbers and relate them to mass human tragedy that stifles our ability to act. It’s not that we are insensitive to the suffering of our fellow human beings. In fact, the opposite is true. Just look at the extraordinary efforts people expend to rescue someone in distress, such as an injured mountain climber. It’s not that we only care about victims we identify with–those of similar skin color, or those who live near us: Witness the outpouring of aid to victims of the December 2004 tsunami. Yet, despite many brief episodes of generosity and compassion, the catalogue of genocide–the Holocaust, Bosnia, Rwanda, Darfur–continues to grow. The repeated failure to respond to such atrocities raises the question of whether there is a fundamental deficiency in our humanity: a deficiency that–once identified–could be overcome.

The psychological mechanism that may play a role in many, if not all, episodes in which mass murder is neglected involves what’s known as the “dance of affect and reason” in decision-making. Affect is our ability to sense immediately whether something is good or bad. But the problem of numbing arises when these positive and negative feelings combine with reasoned analysis to guide our judgments, decisions, and actions. Psychologists have found that the statistics of mass murder or genocide–no matter how large the numbers–do not convey the true meaning of such atrocities. The numbers fail to trigger the affective emotion or feeling required to motivate action. In other words, we know that genocide in Darfur is real, but we do not “feel” that reality. In fact, not only do we fail to grasp the gravity of the statistics, but the numbers themselves may actually hinder the psychological processes required to prompt action.

A recent study I conducted with Deborah Small of the University of Pennsylvania and George Loewenstein of Carnegie Mellon University found that donations to aid a starving 7-year-old child in Africa declined sharply when her image was accompanied by a statistical summary of the millions of needy children like her in other African countries. The numbers appeared to interfere with people’s feelings of compassion toward the young victim.

I think a related explanation for our insensitivity to mass suffering (but sensitivity to the plight of a single individual) comes from the personal/impersonal moral distinction. This distinction is best demonstrated by the infamous trolley example, coined by Judith Jarvis Thompson in the early 1970′s. Subsequent studies have shown that this fuzzy moral distinction is built into our brain. Most children begin to separate personal transgressions (like hitting somebody) from impersonal transgressions (like talking in class) around the age of three. When asked why acts of personal violence are wrong, young kids will refer to the harm done to the individual. Getting hit hurts. However, when young children are asked why impersonal moral errors are wrong, they typically refer to the social disorder that will inevitably ensue. You can’t talk in class because the teacher is talking. You can’t cut to the front of the line because everybody has to wait their turn.

This elementary distinction has important consequences. When kids are told that it’s now permissible to hit your friend, they still believe that it’s wrong. The act of violence still feels bad; our conscience is steady. In contrast, when the rules are changed for impersonal moral violations, they cease to be violations. If the teacher says it’s okay to talk, then it must be okay to talk. The transgression is no longer immoral.

If Slovic is right, then cases of mass genocide, when the individuals are obscured by reams of terrible statistics, fail to trigger the specific parts of our brain that respond to personal moral situations. Instead, the brain treats the situation as an impersonal moral transgression – a case of social disorder – and doesn’t muster the necessary impassioned emotions. The statistics distract us from our innate moral instincts, which make us sympathize with the plight of an individual. We know, of course, that the slaughter is wrong, but it doesn’t feel as wrong as it should. Our moral emotions evolved to respond to the suffering of an individual person, not the suffering of an entire people. Mother Theresa was right: “If I look at the mass I will never act. If I look at the one, I will.”

Perhaps this is why I was more moved by What is the What, the fictionalized memoir of a Sudanese Lost Boy, than by any single news story I’ve read about the Sudanese conflict over the past several years.