Good point -- you have to take into account technological, genetic, geographic, economic, geopolitical, etc. conditions as well.

(Which poses an interesting question: what sort of thing is America or any one of its component parts to be compared to? Or is there a more general rule -- something with a similar structure to "if the vast majority of other civilizations would disagree up to their declining period, you're probably wrong"?)

Steppe hordes, sea empires, and hill tribes may be alike enough that similar preconditions for civilization would be necessary. (cf. hbdchick's inbreeding/outbreeding thing, esp. the part about the Semai: same effect, totally different place)