As has already been pointed out, if you expect the link to be weaker in different societies, than the link is caused by society.
It could also mean that the link is weakened by different social conditions to different extents. It could be a selection effect on the kind of people who move to Vancouver.
In any case, even this statement has politically incorrect implications, namely that we shouldn't be considering Black/Ebonics culture as equal in value to mainstream culture.
By "caused by society" I was referring more to a lack of role models, higher likelihood of a low-income upbringing, alienation due to societal discrimination etc.
However, I would indeed claim that any culture that is encouraging violent tendencies and discouraging academic success should be improved by any means possible.
Here's the new thread for posting quotes, with the usual rules: