Does body posture change your attitude?

If someone is happy, they may have their shoulders back, chest out, head up, arms open and eyes ahead .
What is someone is quite sad, then adopts that happy posture?
Will that make them happy?

I think that adopting a happy posture when we are sad helps our mood. I'm not sure why. Maybe it releases endorphins or something. It's probably a small amount of help though. Worth a try.