A clenched jaw, tightened neck, or furrowed brow shows stress

All these are "limbic responses" associated with the limbic system in the brain.

"Emotion, spotting and reacting to threats, as well as assuring our survival, are all heavy responsibilities of the limbic system," writes former FBI counterintelligence agent Joe Navarro.

"The bus leaves without us, and we are clenching our jaws, rubbing our necks. We are asked to work another weekend, and the orbits of our eyes narrow as our chin lowers."

Humans have been displaying discomfort this way for millions of years, Navarro says.