On behavioural patterns and the pigeonholing of personalities

Some people would argue that people in general tend to behave in a pretty consistent way. No matter how situation-bound one's behaviour might be, there need necessarily be some things that holds true for you in most situations and over time and space.
These traits we call personality. How does one acquire this mysterious thing and is it apt to change over time and/or environment?

If I had answers within unreasonable boundaries of certainty, would these questions be presented in this clearly musing fashion? Certainly not.
Since many of these questions converge to or prove to be a subset of the whole spectacular question of free will (which looks grim), it oftens seems as if they are part of a bigger picture.

Old habits die hard. It also seems that old environments have a tendency of triggering old behavioural patterns. As one gets older, how hard is it not to teach an old dog new tricks. I.e. un- and relearn.

If personality has a basis in culture, that is environment, one could then make the argument that a change in language brings with it a certain amount of (if quantifiable) change in personality.
Too bad we don't have instruments for measuring personality. Sadly, phrenology is here a lost cause, no matter how interesting it might seem to be.

One thing that I have discovered (actually read about, mused and accepted) is how much of our perception is governed by what we expect to perceive.
This also applies to how we expect persons to behave, and this is after having not recieved clearly enough substantial information about them. After recieving something that supports our initial beliefs about a person we tend to discard other, bigger pieces that falsify the whole theory. "Make a decision and stick with it" - also seems to apply to personal relationships.

How this relates to the principle of pigeonholing individuals, that is sorting people into discrete categories, is not hard to see.

We do it automatically and without thinking much about it. That's one of the things I love about evolution [1] (provided the theory is true). Can you tell whether you are going to like a person after having just spent a few minutes with them? Most can.

With these homogeneously human traits one might feel inclinced to draw the conclusion that these levels of abstraction are, and have been, a necessary condition for the forthgoing of the human species.
It is, however, sad it is to see a few layers of rationality skipped.

Notes:
1. A later post might address a topic from evolutionary psychology. These wild ideas, you see, are frequently recurring.