In the 1970's, someone decided to do a study of college professors. The setting for the experiment was a lecture hall in which each professor was giving a speech in front of a large audience of students. And all of the students in the classroom were participants in this experiment. (I wish I had been in on this kind of thing at least once when I was in college.)

The professor stood while lecturing, and walked back and forth, addressing both sides of the lecture hall. The students on the right side of the room were instructed to nod in understanding with the professor when he was looking at them. The students at the left side kept their heads still.

You can probably guess was happened: it wasn't too long before the professor was anchored at the right side of the room, speaking to the nodding students.

I know a lot of people who find fault like there's some kind of reward for it. But people don't crave criticism; they crave positive reinforcement. It's a powerful tool, and yet in comparison with its less effective counterparts, it's astonishingly underused.