In 1974 Ekman & Friesen designed one of psychology’s first non-verbal communication deception experiments where ER nurses were asked to watch positive (such as a happy or uplifting scene) or negative (such as live amputations and burn victims receiving treatment) film clips, and describe one of the gruesome scenes as pleasant to an interviewer. The nurses were filmed, and their films were shown to a large range of professionals. After observing baseline footage of the nurse’s normal behaviour, the results indicated that when the viewer saw just the face of the nurse, the untrained lie detector had a very low chance of correctly picking deception (50 / 50). On the other hand, the same untrained lie detectors became much more accurate (around 50% – 65%) when they were able to see the body language of the nurses.
This suggests that no matter how good someone is at reading faces, being able to see the body will increase your chances of detection. With the body, the non-verbal leakage acts like a gestural “slip-of-the-tongue”.