Psychology very much recognizes that individuals have certain tastes, tendencies, and the like. We can't know everything about people from just percentages.
Still, the structure of our minds is very similar. We all learn in much the same way for instance...classical conditioning, operant conditioning, imitation...these are all ways in which humans learn. Most of the ways in which you act when feeling an emotion are learned through imitation, for example. This doesn't mean you can't stop acting violent when angry, or whiny when discomforted, it just means that you most likely picked up these behaviors through watching how someone else acts. Some introspection will even show you this is true...sometimes when I'm playful in a jovial way I take on the voice and mannerisms of a friend I had in college.
Of particular interest of mine is the cognitive biases. It's absolutely fascinating how many of these people commit and how, even if you explain how it's a bias or fallacy to them, they still don't care and think they are using good reasoning.
Well of course the actions of a person are very dependent on upbringing! If you show mercy toward your peers and they reward you by taking advantage of you every time, you learn to be ruthless. If your father gets angry and violent often, you pick up on that behavior (either by imitation or by associating violence with a bad feeling and so abstain from or even shun it).
Learning doesn't stop when we become adults though. We are not just products of our upbringing or genetics.