You tested it, i doubt you authored it. A paternal twin can have one with good impulse control and another who's more instinctual. Even in the same family and upbringing.
Everybody can be taught some degree of behavior even if it may require different amount / type of effort, I refuse to believe it ain't true apart from most extreme cases.
Sure it happens, but it's unlikely.
It's why kids from certain cultures achieve better academic success on average than kids from other cultures, on average: the environment matters a lot.