Most of us have heard of the "effects of embodiment," that is, the theory that holds that we can fake our way to an emotion. You know, how smiling—even when you feel like throttling another person—makes you feel, well, happier. Somehow, the way you arrange the muscles in your face releases the right neurochemicals. (In at least one study, the subjects were not even smiling, they were just biting a pencil!)
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So what did they find out? Same deal as the smile study. People assuming power postures felt more in power. AND testosterone levels of subjects—regardless of gender—went up while cortisol levels (the stress-related hormone) went down within moments of assuming the power postures. Incidentally, people in power have lower level of cortisol (probably because they're busy raising everyone else's).
On the flip side of the coin, taking submissive postures, which entails making yourself seem smaller (slouching, slumping shoulders, looking vulnerable, hugging self and crossing ankles), resulted in lower testosterone and higher cortisol levels.
So the lesson is, act like a bully. You'll feel good about it later. It's also a great way to build up your confidence right before a tough meeting, an interview, or your next mime performance before a large audience.