Researchers have found that some people facially express pain more than others do. Facial pain expressions are automatic, just like those of happiness, anger, and sadness. Pain triggers certain muscle groups in the face to make certain expressions.
The research conducted at the University of Montreal demonstrated that facial expressions appear to accurately reflect levels of pain. People that expressed more pain through facial expressions reported actually feeling more pain than than others, even though they were all exposed to the same painful stimulus in the study. The investigators suspect that people with greater facial expressions of pain have lower pain tolerances.