Eighty-five per cent of them said it would be morally wrong to push one person off [a bridge] to save five [from a trolley], whether these people are brothers or strangers, confirming the idea that there is a rule against killing. However, despite thinking it wrong, 28 per cent said they would still push a stranger off to save five, while 47 per cent said they would push a brother off to save five brothers. (more)

One of the study’s authors offers an explanation:

Social cohesion demands we have rules, regardless of what they are, to help resolve disputes quickly and peacefully. DeScioli says our rule-making system is arbitrary, producing the belief that masturbation is “bad”, for instance.

But why resort to randomness when other good explanations remain? We naturally want simple clear social norms against murder. While simple rules create unfortunate incentives in specific cases, they are overall easier to monitor and enforce. This trolley problem seems to be one of those specific cases where many of us think that our simple rule against murder goes wrong – while we agree that killing in this case violates our murder norms, even so many of us are willing to violate such norms in order to help associates, especially if we care a lot about them.

While morality may be in general pro-social, it is not in every specific case. So there are times when you must choose between being moral, and being helpful.