Behavior is determined by breed and individual temperament. It's also influenced by flock dynamics. I would opine it may be as complex as human behavior, although in different ways of course.
A lone chick will be instinctively craving to belong to a flock, while at the same time being intimidated by the very same flock she craves to be part of. Chickens are influenced by their individual temperament to be timid or assertive or somewhere in between, and that influences how quickly they assimilate into the flock.
I like to take things slowly, watching to see if I can determine what kind of temperament the individual has that I'm integrating, and I move things along at a pace to preserve her self confidence. If you simply toss an individual into the flock and she happens to be timid, the flock picks up on that and the timid one then may become a chronic victim. We want to avoid that.