Body languages (including postures & gestures) are quite integral part of clinical psychology; and I believe it's quite important for photography too. Body language is the language of the body. We may think that we are only showing our emotions through our face; but that is, quite literally, only the tip of the iceberg. Our entire body, through it's postures & gestures, participates in the business of either showing or hiding our mental state.
Body language is extremely visual; observing and capturing those cues through photography can be analytical. In this context, I feel "streets" should be considered as "bedside medicine" of clinical psychology to "observe" the body languages; it's a fun to learn. I personally consider street photography not just to capture 'decisive moments', rather streets are the ultimate place to decode the non-verbal communication among humans species, visually. I call it as Street Language of Psychology!!!