It always has been split down the middle...simply because it is. Science has never been able to determine if nature is more important than nurture and have done extensive studies for many years~still arguing this as far as I know.
It's probably wise to view these things on a case by case...
I would say it has a lot to do with it, but the old nature vs. nurture argument still rages in many circles, animal and human alike. I've never had an aggressive roo in all the years I've had flocks and I fail to believe it was just because I got lucky...over and over again.
I think both...