You see a person mugged in the middle of a crowded city block. You come upon a motorist stuck on the side of a rural road. Your boss makes a racist joke in the lunchroom. A bag breaks and a harried dad’s groceries spill onto the sidewalk.
Do you act? It depends on how your brain calculates the costs.
A classic study by psychologists John Darley and Bibb Latané found that the more witnesses there are to an event, the less likely any one person is to help. This, they said, is because the responsibility for the person in need is spread over all the onlookers. If you were to help, your individual costs in terms of time, effort, risk, etc., would outweigh the fraction of the total responsibility that is yours. This means that a motorist may have a much more difficult time flagging down help on the side of a busy freeway than he might on the Alaska Highway. In the first case, a passing driver can expect that someone else will stop; in the second case, the responsibility is on one driver alone.
But lest ye prematurely chalk humans up as cold, cold machines, don’t forget to factor empathy into the calculus of compassion. The more connection you feel with the person in distress, the more it costs you to simply walk past. Thus when a harried dad spills his groceries on the sidewalk, another dad would be most likely to stop. (And with more people watching, there’s more likely to be another dad in the audience—so a crowd’s increased chance of including one person who empathizes may outweigh the decreased overall responsibility of each person in the crowd.)
What about the crime in broad daylight? There are many potential costs of action. Again, costs include risk, time, and effort. And there are risks of inaction: guilt, blame, further crimes. (Can you plausibly deny knowledge that a crime occurred right in front of you? Can you assume the police will catch the perpetrator without your help?) And there are the rewards of heroism, both external and internal.
The values of these factors create our choice.
And they do it in a blink. In the second before whizzing past, we notice if the hitchhiker is a derelict or simply a distressed motorist just like us (empathy). We notice if the pickpocket looks dangerous (risk). We evaluate how likely the problem is to be solved without our help (responsibility). We think of the $500 for returning the lost dog to the attractive lady (rewards).
Our brain runs the numbers and then we act. Or we don’t.