Personality develops itself from birth.
Around half influenced by the genetic mix of the parents and half
influenced by the social norms and environment.
Around half the variations in personality, intelligence, and behavior comes from something in the environment.
But whatever that something is, it cannot be shared by two children growing up in the same home with the same parents
The big five personality traits, OCEAN are heritable at around 50%.
- Openness to experience
The older we get, the more stable the personality becomes.
As a snowball effect, we start with a set of big traits and express them through the choices and oportunities that open themselves in front of us during life, plus a bit of luck twist.
It is also shaped by the norms around us, we all want to climb the social hierarchy, whatever it currently is.