Why do people nod their heads up and down if the professor is talking directly to them?

It’s an active listening skill that, along with leaning forward and giving verbal acknowledgement (“mhmm,” “I see”) respectfully indicate to the speaker that you are engaged in the conversation. We all probably learned these skills naturally, as they enhance conversation.

Another possibility: human brains are wired with “mirror neurons”, neurons that respond as a mirror to behaviors observed in others. When mirror neurons fire, your body is likely to respond as if you were performing the action you are observing.

So, for example, if your professor says something and nods his/her head, your mirror neurons would fire as if you were nodding your head, too. Similarly, if others in the class were nodding their heads, your mirror neurons would fire as if you were nodding your head. And these things would increase the likelihood that you would nod your head.

pretend

What tonedef said.