I’ve always thought it would be a combination of both and that was confirmed at a workshop I attended a number of years ago (and rediscovered when I cleaned out a drawer recently). Dr John Barletta, clinical psychologist, presented the workshop which examined how both nature and nurture can influence the development of behaviour and personality.
I loved the following quote from Dr Barletta’s presentation:
It doesn’t matter where you plant a pumpkin seed, it will always grow into a pumpkin. But how big, hardy and tasty it will be depends upon…? As our nature is a type of genetic endowment, nurture is the experience we have during our life time.
How does this relate to writing? I see it as being very strongly related to a character’s behaviour and personality – and quite possibly the basis for the formulation of the internal conflict: It is a fallacy to believe that any behaviour that is genetically inherited cannot be modified over a lifetime…
So what has influenced your protagonist throughout their early years, combined with their genetic inheritance, will be what forms them as adults. And it is this behaviour they have to overcome to reach their Happy Ever After.