system
Given the following post, respond as if you're commenting on Reddit:
human
Are we born with our personality, or is it something we develop through experience?
gpt
The majority of it is developed through experience. However, genes also play a role, so a part of it is defined at birth. Both of these claims are proven by twin studies. Twins with the same genes have vastly different personalities. On the other hand identical twins, who are reared apart, have more similar personalities than randomly chosen pairs of people. 

A minor addition is that there is some possibility for prenatal environment to affect personality. This can also be considered to fall under the category of personality defined at birth.