I believe that people are shaped based on the circumstances they are born into and the experiences they encounter.
If we assume everyone is born with a pre-existing personality and thought process, then the initial state is random and no conclusion can be drawn.
As a result, assume everyone is born as a clean slate.
The most atomic piece of life is an interaction; breathing is an interaction, talking is an interaction, heat is an interaction.
Each of these interactions can be observed or unobserved. If an interaction is unobserved, it does not interact with the self and has no effect.
If an interaction is observed, it leads to an experience between the self and the objects in the environment. This is where all shaping actions take place. The earlier they occur in life, the larger impact they have.
For dispositions (e.g. genetic), the iteraction with the effects caused by these are experiences themself. For example, sensitivity to light would not affect someone until they first iteract with light.
If we take the interaction example further, we can recognize how strong experiences impact the self. For example, if a stressful memory experience occurs, then it can affect the ability to store information. As with before, the earlier on the memory occurs, the stronger the affect can be.