Twins per definition have the same social and genetic background, so any difference in IQ cannot stem from those factors.
The most interesting Twin Studies has 'twins separated at birth' studied so that there was an identical genetic background but a different
social background, as they're brought up in different families.
But it's harder and harder to carry these out, these days, given we don't tend to do this quite so flippantly separately adopt twins; even if we might now at least more easily track/re-acquire the pairs, rather than them getting lost in the system until they both end up retiring to the same Florida retirement home in 60 years time and noticing they tend to buy the same wacky type of tie and, come to think of it, look so much alike that they've already been unintentionally confusing friends and family over many years, and that's why they were both temporarily sectioned in the '80s, ironically in different wings of the same hospital...
Darn the new social conscience, eh? Look what information we're losing!
The results showed no significant difference in IQ between a twin that smoked weed during adolescence, and one that did not.
In a same-family situation, one wonders whether a
cause for the twin-that-smoked-weed was discovered (or else, a reason why the twin-that-did-not-smoke-weed did not).
Are you more likely to regress into the arms of the spliff if everyone, especially your twin-sibling, keeps reminding you that you're the
younger twin? Or, of the two twins, you're the one with the silliest name (your brother is Bertone, and you're Berttwo).
Science demands to know! No, scratch that,
I demand to know... Science (and my long-lost secret twin brother who is almost definitely a billionaire with his own metal suit, or so I extrapolate from all the available evidence) probably already knows but they refuse to tell me!