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That book I recommended you read, you said you read part of it? Talked about this very same thing.
S-curves...and the waves..

I read it - but kinda skimmed across the last parts.

I never saw any IQ/genetic argument...? did I miss something (page no's?)

The S curves were a interesting (and I think accurate) representation of progress within societies. Not IQ variances.

I think you might have missed the point of the book.
it was descriptive. The author noticed that 'certain things happen' and was pointing them out.

He did NOT delve into why anything would happen only that it did...and how to recognize what was happening and to be prepared...
......or not.