Often both are involved. There are genetic markers that will be found to cluster in families with addiction that have direct links. Having those genes isn't necessarily destiny, but the heritability and activation even in separated twin studies or adoptions underscores the biological component.

Conversely, adverse or traumatic life events, periodic toxic stress, severe interpersonal or behavioral issues can also lead to addiction in individuals lacking a genetic or familial component (all of these conditioned or 'nurtured' elements have a direct material trace on the brain, especially the developing brain).

If you have both elements coming at you, it's very very difficult to cope with or overcome substance abuse on your own.