They asked subjects to make a series of decisions about how many moderately painful electric shocks to deliver to themselves or to others. (sciencemag.org)
Milgram, who was already famous for the obedience experiment in which study subjects administered painful electric shocks to other study subjects when urged to do so by an authority figure, came up with the letter method as a tool to try to solve the problem in real life. (discovermagazine.com)
That was, of course, to avoid painful electric shocks from the packs affixed to their buttocks and activated if they failed to show said affection. (newscientist.com)