Dawkins (and I) believe Darwinian natural selection has ingrained some "moral universals". These "deep structures", like our inherent capacity to learn language, may vary a bit from culture to culture, but are evidence of a natural sense of right and wrong. The trolley example is designed to tease out this basic sense.
The traditional example assumes a trolley is running amuck along the main tracks and will kill five people if it is not switched onto a siding or stopped in some way.
1) A moral person, Denise, is standing by a track switch and could divert the trolley from the main line to a siding. Everyone would agree Denise should throw the switch to save the people.
2) However, there is one man on the siding, and he will be killed if she throws the switch. (Assume Denise does not know any of the potential victims and there is no time to warn them, etc.) What should she do? Should she kill one innocent person to save five innocents? Write down your answer and proceed as the example gets more and more difficult.
3) Alternatively, a moral person, Ned is on a bridge over the trolley tracks. If he could throw a large weight off the bridge onto the tracks, that would stop the trolley and save the lives of the five innocents. Everyone would agree he should throw the large weight off the bridge to save the people.
4) However, the only large weight available at the moment is a very fat man resting near the low railing and in a perfect position to be dropped to the tracks. Ned is strong enough to push him over and the man is certainly fat enough to stop the trolley. What should Ned do? Should he kill one innocent man to save five innocents? Write down your answer and proceed as the example gets more and more difficult.
5) Alternatively, a moral person, Oscar is standing by a track switch that could divert the trolley to another line. A large weight (say an empty stationary trolley) is parked on that line and would certainly stop the runaway trolley and save the innocents. Everyone would agree Oscar should throw the switch and have the runaway trolley crash into the large weight to save the people.
6) However, there is a hiker on the other line. Unlike the fat man thrown off the bridge, his body will not be used to stop the runaway trolley, but he will surely be killed. Should Oscar kill the innocent hiker to save five innocents? Write down your answer.
If you think Denise and Oscar should act, but Ned should not, you are with the vast majority of people surveyed. That this is a moral universal is attested to by the fact there was no statistically significant difference on this issue between religious and non-religious people. An analogous problem, featuring crocodiles and canoes, was posed to primitive tribesmen in Central America with similar results.
I'd appreciate discussion of why you think Ned should spare the fat man and condemn to death five equally innocent people. Why should Denise and Oscar "play God" and condemn one innocent to save five? Do you agree with Dawkins and me that there are certain "moral universals" that have been hard-wired into each of us by Darwinian evolution?