[themk]

This sounds like they didn't think about it at all. Of course they did, and sure, their techniques were not as sophisticated as today. But there have been plenty of follow-up studies that have controlled more rigorously for those things, and it turns out they were probably right?

Also, the 7 countries study didn't just compare the regions, they also did intra-regional comparisons. Not that I think this particular study is what you should base all your evidence on, but, most others back it up.

The people who run these studies actually know what they are doing. They know the limitations of their methods, and, they have thought about confounding variables. This _always_ comes up in internet debate, like, "ahh, but there are confounding variables so the study must be trash!". It's literally their job to take those confounding variables into account. They don't just grab random people of the street to run these things. And I assure you, they know about the details.