Imagine you are a train-yard operator who sees an out-of-control boxcar running down a track that five workers are busy repairing. The workers won't have time to get out of the way unless you flip a switch to change the car to another track. But another worker is on the second track. You have just seconds to make a decision: let the five workers die -- or kill the one. What do you do?
This dilemma is a famous philosophical conundrum that was originally called the "trolley problem." Now a team from Michigan State University's psychology department has used virtual-reality technology to test how we respond psychologically and physiologically when faced with this problem. (See TIME's health vertical, Healthland.)
The two opposing philosophical approaches to the trolley problem are the utilitarian one (kill one guy in order save the others) and the do-no-harm approach (let god or nature take its course, but don't make an active choice to kill another person).
Finally -- and most importantly -- when the one person you would have to kill to save five is your child, parent, or sibling, only approximately one-third of us will opt to protect the five people.