I remember losing a debate because of a head shake.
It was an oral argument in law school, and I had just finished giving a presentation when my opponent started speaking. I found myself pleased that I had anticipated a number of the arguments that my opponent was making, and without even thinking about it, was smiling and shaking my head when she made the points she did. Not because I thought she was right, but rather because I believed that I had addressed those issues.
I thought I had made the stronger argument. Surprisingly, the judge of the event told me that I had lost points and credibility when he saw me shaking my head at the statements my opposition was making, as if I was agreeing with her.
Credibilty can be defined as a mixture of expertise and trustworthiness. The words you use, whether in conversation or in writing, can send a message that you aren't aware of. So can your nonverbal language.
I enjoyed this quick list of words that could also influence whether or not people believe you, and trusted what you are saying during a presentation, or negotiation, or confrontation, or even an interview: 52 Words and Phrases That Weaken Your Credibility
If you find yourself in that type of situation, where you have an audience tryng to determine what message you are sending, keep in mind that your expression, your motions, and your words are all the clues they have about what you are presenting. Even when you aren't the one talking. Credibility doesn't just rely upon what you say, and how you say it, but also how people perceive your message, your demeanor, your appearance, and even the way your head shakes.