The very phrase has become something that may cause revulsion, or squeamishness in the modern reader. The phrase brings up ideas of soldiers killing innocents, the Nazi torture of the Jews, and any other untold number of atrocities that have been afflicted upon others. I think one poignant remark made in a movie was by Magneto...
This is certainly a phenomena that can lead to atrocities, but I think we all too frequently are unaware of our own ability to fall into this trap. This unawareness I think is dangerous. Thus today I will talk a little about the psychological research that has gone into this subject.
Milgram
Milgram's experiments were some of the most shocking and most telling about how rampant this phenomena actually is. Milgram, a Jewish professor, was shocked in hearing the defenses given by Nazi soldiers during the Nuremberg trials. Thus he formulated a study, he wanted to see just how obedient a normal person would be if their obedience meant harming another person. In order to test this Milgram had two participants, a confederate "learner" (person who is in on the study), and a "teacher" who was being studied. The "teacher" was told that they were studying the effects of pain on memory. The teacher was asked to shock the "learner" each time they gave an incorrect answer and subsequently raise the voltage of the shock with each incorrect answer. 
The teacher started off administering a "light shock" of 15 volts and incrementally increased to 375 volts which had the warning "severe shock) written by it and then up until 450 volts with "XXX" by it. If the teacher got squeamish or refused to continue shocking the experimenters prodded four times "the experiment needs to continue." In this study 100% of respondents continued to 300 volts and 65% continued to the lethal shock level.
This information was deeply troubling and interesting to researchers. As a result hundreds of similar studies have been conducted researching the various mechanisms involved and what sort of conditions would increase or decrease obedience. I'll just mention a few, but if you'd like to look up more they're all quite interesting. One factor is whether or not the teacher perceives the "leader" as an authority. In one study if the person prodding the teacher was not wearing a uniform then obedience greatly dropped. Similarly when the proximity of the teacher and learner decreased disobedience increased. Basically, when the teacher could see and touch the learner their willingness to obey dropped by a third. Lastly, if instead of doing the shocking themselves they were able to delegate punishment (telling another person to conduct the shocks) obedience skyrocketed with ~95% shocking the learner at lethal levels.
Why?
As stated before, when we hear about these sorts of notorious acts we like to think that we would act differently. We like to think that we would stand up against the crowd, or in this case the individual (next myTake may be about the crowd). Yet the research clearly indicates that this is not likely the case. Why is it not the case? How can we rationalize causing harm just because we're told to? Psychology has also given us some interesting answers to this question. The largest factors appear to be:
1.) Foot-in-door, this phenomenon is that once we begin to listen to an authority we feel compelled to continue doing so. This phenomenon is a compliance tactic that involves getting a person to agree to a large request by first setting them up by having that person agree to a modest request.
2.) Loss of agency, this particular excuse/ response is a process in which we appear to no longer consider ourselves the actor, but in fact we are merely an extension of the person commanding us. Many of the respondents in the Milgram experiments were actually quite angry with the researchers, and would shift the blame to them saying things like, "if they get hurt/ die it's their fault."
3.) Justification is the process by which we try to condone our actions, perhaps we're causing harm but there's a greater purpose.
4.) Society teaches us from an early age to be obedient, particularly to authority figures. In my estimation this may be the most important factor in obedience. Children are often taught not to question their authorities, "they're an officer, do what they say," "the doctor says to take the tylenol," etc. This is necessary for the functioning of society, but can obviously lead to negative consequences depending on the character of the authority.
As always, criticism is encouraged.
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