location => zip code. Twin studies are not what you look at to prove (or disprove) this hypothesis. You have a much richer data set. All the kids that grew up in a zip code. A lot of twin studies have no control groups! With a richer data set, you can more easily look at the zip code someone grew up in and look at where they end up in life. You don't have to track them. Also, the reason why I insist on focusing on zip code is that peer groups matter much more than family environment.
Below, I've linked some of the articles I have read on this in the past.