I know I've told this story before: I grew up being friends with two brothers who were just over a year apart in age. Lived in the same family. Great parents. Lots of love. Both really smart, both did really well in school and in sports.
One of them ended up being an elite athlete (and now runs his own fitness business on the East Coast). The other became a drug addict and died in a house fire before he turned 30.
It's not science, but it's the one piece of evidence I have that there has to be some kind of physiological component to addiction, when two kids from the same family, with such similar circumstances up to the age of 18, can end up going down such different paths.