Most people say they would let a runaway trolley kill one person to save five others if all they had to do was flip a switch, but not if they had to push that one victima difference that leaves philosophers scratching their heads. Now scientists may have found a psychological explanation: humans appear to exhibit a greater emotional response to the up-close-and-personal pushing case.

Joshua Greene and colleagues at Princeton University asked people to make a number of decisions while their brains were being scanned by an MRI machine. The results, published in Friday's issue of Science, indicate that the subjects used the parts of their brain associated with emotion much more when they were asked to decide whether to push someone in front of a trolley or off a lifeboat. When faced with similar but more impersonal dilemmaswhether to flip a switch to divert the trolley or vote for a law that would kill a smaller group of people instead of a larger onetheir brains showed greater activity in places associated with working memory.

"Theyre very similar problemsthey seem like they are off the same pagebut we appear to approach them in very different ways," Greene remarks. Emotions dont necessarily rule, though. People who said they would push that one person in front of the trolley took longer to make their decision than those who wouldnt do it. This hesitation could mean they were going against an initial gut reaction not to push.

Greene's research is part of ongoing trend in moral psychology that seeks to explain decision-making without resorting to traditional rationalism, in which moral decisions were thought to be the product of cold reasoning. As coauthor John Darley puts it, "Moral issues do not come to you with a sign saying Im a moral issue; treat me in a special way.