I never do this, but then I'm an exceptional social retard.
I've also never noticed anyone doing it to me, for example in sales environments etc...probably due to my complete physical non-reaction to just about anyone.
You might think that, but it's not something you notice, unless you're actually looking for it specifically.
Even the most socially guarded individuals convey signals to those around them. Often when I've felt ill at ease in a situation, or unaware, the way you stand or hold yourself (fnar fnar) gives away to others if you're approachable or not in that time/place etc.
Again as all of this is largely subconscious behaviour, unless you are looking for it and know what to watch for with others behaviour (and therefore your own too), you can miss, on a cognitive level, a lot of information, never mind take advantage of it actively.
Everybody does it so some extent - it's easy to see in close knit groups and friends - down the pub, school kids, students, at work. There's superficial stuff, like the way kids dress in similar clothing, talk the same way and use slang, for example. How relaxed and open in speech, posture and eye contact and facial expression is plain to see with people who know each other well.
Then again you can see it in groups of strangers too. Next time you're in a waiting room, see how people sit and where they choose to sit. If it's a doctors waiting room, chances are everyone will keep to themselves and avoid talking or making direct eye contact, arms folded or legs crossed perhaps (you won't see them sprawled out like they would with some mates, watching the tv) - unless there's something external that brings a common bond, most people would be somewhat uncomfortable (whether they show it or not) if you sat right next to them and struck up a conversation
Direct eye contact is one of the clearest indicators of intent or openness, followed next by posture (the way you sit or stand, defensive or relaxed, attentive or aggressive). There's nothing really magic to it. We've all seen the guy trying to impress a girl, and see that she's clearly not having any of it lol he's pulling out all the stops and she's just thinking 'no way buddy, who is this jerk?'. Usually, he's drunk and being overbearing, and she's looking for an excuse to reclaim some personal space
That's the crux of it right there, individual and group social behaviour that states whether you are a threat/mate/ally/submissive/dominant etc. We all do it, we can't
not do it, but most of the time we overlook it and only notice these signals in the most extreme examples, such as aggression and fear or attraction. There's a lot more to it than just these, many more subtle, but like common sense, we know it, but don't know that we know it
