In his book Studies in Expressive Movement, famed psychologist Gordon Allport and his co-author Phillip Vernon wrote that there is consistency in people's expressive behavior, and that personality can be perceived from nonverbal cues. Introverts and extroverts practice different body behaviors, as do Type A vs. Type B personalities.
Your habits, gestures, movements and facial expressions can reveal a lot about your personality, including the way you clench your fists. Quick — make a fist! Now look down and check your thumb placement, and use the link below to find out what it says about your personality!
I got "B", and it matches me to a point.