This is the nature(genetics) vs nurture(environment/culture/upbringing) debate.

Most scientists believe both of these to play a role and there's fascinating interplay between them.

One interesting new field is epigenetics.

Schizophrenia is a compelling example. 

People may have the latent genetics for Schizophrenia and never display it.

However, certain environmental pressures can cause these genes to express the phenotype and that person has these genes expressed..