But American philosophers have usually held a more jaundiced view of crowds. Henry David Thoreau wrote in his journal, “The mass never comes up to the standard of its best member, but on the contrary degrades itself to a level with the lowest.” Ralph Waldo Emerson wrote in “The Conduct of Life”: “Masses are rude, lame, unmade, pernicious in their demands and influence, and need not to be flattered, but to be schooled. I wish not to concede anything to them, but to tame, drill, divide, and break them up, and draw individuals out of them.”

These thinkers argued that crowds add up to something less than the sum of their parts. The principle behind this is called “deindividuation,” in which an individual’s social constraints are diminished and distorted by being part of a crowd that forms to express a particular point of view. The French psychologist Gustave Le Bon first explained this concept in his magisterial 1895 text “The Crowd: A Study of the Popular Mind.” Le Bon found that crowds were inherently “unanimous, emotional and intellectually weak.”

Lots of research confirms this, showing that deindividuation can lower inhibitions against immoral behavior. In one of my favorite studies, researchers set up a bowl of candy for Halloween trick-or-treaters, told them to take just one piece and then left them alone. Some of the children were in anonymous groups, others were by themselves. When kids were part of a group, 60 percent took more than one piece of candy. When they were by themselves but not asked their names, 20 percent cheated. But when they were alone and asked their names, only 10 percent took more than they were allotted.

Of course, it stands to reason that deindividuation could improve individuals instead of making them worse. We can all think of cases in which we have been swept up in a wave of kindness and compassion in a group, even in spite of our personal feelings. Group polarization, in which individuals are pushed emotionally in the general direction of the crowd, can be either positive or negative.