The entire book is bogus. About 1/3 of the wikipedia article is dedicated to the criticisms of the book, and they seem pretty damning.
http://en.wikipedia.org/wiki/IQ_and_the_Wealth_of_Nations#Cr...
http://en.wikipedia.org/wiki/IQ_and_the_Wealth_of_Nations#Cr...
Of relevance to this forum are the findings on entrepreneurship in the industrial revolution. The people who built the companies and science that made it possible and greatly elevated Britain never made anywhere near enough to compensate their investments of time and money. They were irrational actors. Clark argues they acted this way because of genetic predisposition -- a predisposition that was not widespread in other societies.
'What was being inherited, in his view, was not greater intelligence — being a hunter in a foraging society requires considerably greater skill than the repetitive actions of an agricultural laborer. Rather, it was “a repertoire of skills and dispositions that were very different from those of the pre-agrarian world.”
Also, the article points out that many people are skeptical of his view that the behavioral changes have biological causes and not just cultural. For instance:
'The natural-selection part of Dr. Clark’s argument “is significantly weaker, and maybe just not necessary, if you can trace the changes in the institutions,” said Kenneth L. Pomeranz, a historian at the University of California, Irvine.
It's a related theory, not the same bit on IQ and I didn't mean to imply otherwise.
> biological causes and not just cultural
I know I mentioned the word "genetic", but really all that matters is we're talking about a set of heritable traits; nature vs nurture is kind of moot. That we're talking about something heritable is detailed in the book.
This is a bit of a myth. Very high childhood and teen mortality brought average lifespan down, but once you were 20 making it to 65 was not rare at all. In places like America where plenty of good food was affordable old age was fairly common. Take a walk through an old cemetery and see for yourself.