WHY do we sometimes struggle with moral dilemmas? Why is it a crime to take certain drugs but it's acceptable to take others? Why is it wrong to create human embryos to cure diseases suffered by millions? Often, we follow rules that bring little benefit and can even be positively harmful. But the rules are not set in stone, so there is nothing to stop us getting rid of those that don't work and putting better ones in their place. Now an experiment suggests that morality isn't entirely about benefits to individuals. We also tend to make and obey arbitrary moral rules, probably as a way of promoting social consolidation . A classic psychology experiment called "trolley experiment" suggests that our minds have two moral systems, and they don't always agree. In the trolley experiment, participants are told that an out-of-control trolley could kill five people on the tracks. They must decide whether to turn it onto a second track with only one person on it. Almost everyone does it, sacrificing one to save five. But if instead you have to push one person off a bridge onto the track to stop the trolley, most people say no. That suggests most of us have a strict rule against killing people directly, even for the greater good. How are such rules formed? Although people with morality appear to _ the act of killing in the bridge experiment, most moral behavior in animals appears focused on outcomes--the death of an individual, say--rather than the death of the majority. When an animal experiences harm to help a , biologists view this as increasing the chances that the animal's genes will survive. Many psychologists think that human moral rules are an extension of this "kin selection". Kurzban of the University of Pennsylvania did the experiment further. Kurzban's team gave volunteers changes of the bridge situation. Volunteers were asked what they would do and whether their actions were morally right. 85% of them said it would be morally wrong to push one person off to save five, whether these people are... According to the passage, the rules of morality are, under some circumstances, _ . A) scientific B) reasonable C) harmful D) flexible
D