Sure there are other factors, but I think that's the biggest, and the one with the most explanatory power. Fred Reed wrote a great column on this topic a couple of months ago.
In that article, he references a longer paper which has a lot of comparative detail. Social consequences of group differences in cognitive ability
These articles are about as un-PC as it gets.
I clicked the link you posted and thought briefly of posting it in “chat” but quickly realized that a moderator would soon yank it. Unvarnished truth is a hard sell sometimes.
Thanks for the lead though, I’m going to read some more of his stuff.
The Bell Curve explains a lot to those with the ability to understand what they are looking at and all its social implications.