Peter Collett is famous for his Book of Tells, a fascinating handbook on interpreting body language. It is my current bathroom reading book.
He made a television summary in 2004. Someone has put both parts on YouTube.
• Episode One: Power and Honesty
• Episode Two: Attraction and Flirtation
Noticing how people's internal states are revealed by their body language is fun. Even more fascinating is what I recently learned about the work done by Amy Cuddy about the opposite dynamic: how our posture effects our hormones and feelings.
Her most famous research measures how holding a "power pose" for two minutes changes your hormones: testosterone (causes confidence) goes up by about 15% and cortisol (causes stress) goes down by about 15%.
One aspect of her work seems incomplete. I know from reading Joe Navarro's book on body language that asymmetrical postures seem especially confident because they demonstrate a level of relaxation usually shown only by confident people. Consider three examples of how to improve her sample power postures.
First, the posture of standing like Wonder Woman. Keep one side of the body with hand on hip (thumb back) and foot facing forward. On the other side, hook one thumb into your pants pocket and point the foot away at a 45 degree angle. Your sternum will rotate slightly towards the angled foot. Ta da! Now you look relaxed and slight pensive, instead of stern and about to clobber someone.
Second, the posture of the businessperson faking relaxation: leaning back in a chair, hands behind the head, legs crossed at the ankles with the feet on a table or desk. But instead of crossing the legs at the ankles, rest one ankle on the other knee (so the legs make a "4" shape). Now you look genuinely relaxed, not faking it.
Third, the posture of the person sitting while trying to take up space: elbows up on the chair as high as the shoulders, legs out, knees bent and apart. Make this asymmetrical by keeping one leg straight and the other knee bent, and resting one hand/forearm on your leg to keep your shoulders slightly slanted.
UPDATE: At the local library I discovered a third body language book by Alan and Barbara Pease. Its prose is less readable than Navarro's, but it has many more helpful pictures. (Collett's book is miserable to read: the prose is dry and overly focused on terminology, and the book has very few pictures.)
UPDATE: National Geographic has a fun video about similarities in dominant body language between humans and gorillas/chimpanzees.