Many different rationalizations have cropped up over the millennia – from controlling resources to exterminating races to "fighting terrorism" – but murdering strangers requires a particular obedience to authority.

Fifty years ago, psychologist Stanley Milgram conducted the Milgram experiment, which measured the willingness of study participants to hurt others, even when it conflicts with their personal conscience, in the interest of obeying authority. The results, published in 1963, showed that a very high percentage of participants were willing to follow orders and cause serious injury to others.

One would think that 50 years later – after the lessons of World War II, the horror of Vietnam, the bloody folly of the Iraq invasion, and the long list of brutal military crusades – humanity would have a greater aversion to harming others at the behest of authority. However, a replication of the Milgram experiment shows that people are as willing as ever to obey authority to the harm their fellow man.