I'm not going to try to do much in the way of a short summary. Clark's argument is too rich and too unconventional to subject quick summary. When someone proposes a new way of thinking about human history, we have to work to take it in. Scholars need to argue about the hypothesis; many voices need chip away at the evidence. In time, ideas stand or fall.
Suffice to say, Clark is proposing that in England and Northern Europe in the period 1200-1800 members of the upper classes produced more children than the poor, forcing ongoing downward mobility from the upper classes. This implanted bourgeois values -- thrift, discipline, etc. -- in the general population. These social changes allowed the technical and social innovations we call the Industrial Revolution to result, not as in the past in merely a temporary increase in human population followed by famine, but in the "take-off" that characterizes modern life. The previous long era when economic success led inexorably to overpopulation which in turn destroyed any long term gains in the standard of living is labeled "Malthusian" after the Reverend Thomas Robert Malthus who first laid out this analysis around 1800. Outside northern Europe and its overseas extensions (like us), Clark maintains the phenomenon of upper class high fertility and subsequent downward mobility that created the social prerequisite for a wealthy technical society hadn't happened -- and still hasn't -- so these societies can use modern technology, but this doesn't make for widely shared gains in living standards.
Yes, this is counter-intuitive in more ways than I can list. And also darned interesting. Here's Clark's basic graph of human history:
Note that, in addition to the enormous gains of modern winner societies, there are also modern loser societies. Here's a Clark observation that gives the flavor of the originality and audacity of this book:
Not for nothing was economics called "the dismal science" in Malthus' time.
Do I think Clark's evolutionist explanation of the triggers for a successful modern industrial society is true? Apart of the thesis' many counterintuitive elements, many of which Clark addresses exhaustively with massive evidence from pre-industrial records, there is something that doesn't feel quite nailed down here. I hope this book will be subjected to serious academic consideration, even if the entire thesis doesn't hold up. Bold big history deserves big discussion -- even it is not quite as comprehensively explanatory as it aspires to be.