The failure to help is tied less to acceptance of life on the streets and more to disdain for homeless people, said Caplan.
"People see the homeless as a blight, like spilled garbage. They wouldn't even think to pay attention to whether they were in trouble. Homeless people don't get looked at that way. It's dehumanization and it's terrible."
Wolpe said he hestitates to criticize those who passed byTale-Yax "because though we imagine that we would have stopped, studies show that in these situations, most people don't."
Though we view ourselves as altruistic, when it's time to act, other motivations often get in the way, Wolpe said. They can be as primal as fear of personal harm. Or as simple as being in a rush.
That was the challenge facing Princeton University divinity students who were put to the Good Samaritan test in 1973. The students were asked to give a spontaneous sermon on why people should always stop to help strangers, but were instructed to go to the church immediately to deliver the sermon and not to arrive late. En route, the students passed a man slumped in an alleyway, in a state of unknown distress. Unbeknownst to the students, the man was acting the part for the study.
Though these priests-in-training were about to preach about helping strangers, only 10 percent of them actually stopped to help the man when they were put in a "high hurry" situation.
"There you have it," said Wolpe. "That's the difference between someone's tendency to see themselves as good and how people actually behave when the time comes and their own lives and concerns get in the way."
ABC News conducted its own altruism study in March 2009, enlisting actors dressed as either a homeless man or a well-dressed woman to faint in the street.
While many passersby rushed to the aid of the well-dressed woman, most people ignored the homeless-looking man. The exception was one female passerby who had been homeless herself in the past, and who stopped to see if the man lying in the street needed help.
When interviewed, those who walked past the "homeless" man cited fear of personal harm and the dangerous nature of cities as reasons they failed to act.
"Especially in this area, you don't know," one bystander, Peter McKnight, told ABC News. "People come up with all kinds of scams, all kinds of situations to try and take advantage of being nice. And therefore, you have to be very careful."
This culture of fear helps explain why people might have ignored Tale-Yax, said John List, professor of economics at University of Chicago.
"People systematically think about the probability that the person really needs their help and weigh it against the probability that helping will put them in danger," he said. In a city, people are more likely to suspect that others mean them harm.
"Fear of the other" is instilled in us from childhood on and "makes it hard for people to respond to the needs of strangers," said Wolpe.
Because children are taught to be wary of strangers, they often grow up to be people who do not know when it is right or safe to intervene to help a stranger, Wolpe said.
"I'm not saying we shouldn't be teaching kids not to get in cars with strangers, but it's really important to balance that with an ethic of caring and intervention," he said.