Dr. Clark’s first thought was that the population might have evolved greater resistance to disease. The idea came from Jared Diamond’s book “Guns, Germs and Steel,” which argues that Europeans were able to conquer other nations in part because of their greater immunity to disease.Clearly, there are potential pitfalls one gets into when applying evolutionary-style analysis to cultural questions. And yet, to the extent that these practices and attitudes are heritable (which certainly doesn't mean that they're strictly genetic, just that there's enough heritability for relative reproductive success to change the prevalence of attitudes) it stands to reason that they could be selected for.
In support of the disease-resistance idea, cities like London were so filthy and disease ridden that a third of their populations died off every generation, and the losses were restored by immigrants from the countryside. That suggested to Dr. Clark that the surviving population of England might be the descendants of peasants.
A way to test the idea, he realized, was through analysis of ancient wills, which might reveal a connection between wealth and the number of progeny. The wills did that, , but in quite the opposite direction to what he had expected.
Generation after generation, the rich had more surviving children than the poor, his research showed. That meant there must have been constant downward social mobility as the poor failed to reproduce themselves and the progeny of the rich took over their occupations. “The modern population of the English is largely descended from the economic upper classes of the Middle Ages,” he concluded.
As the progeny of the rich pervaded all levels of society, Dr. Clark considered, the behaviors that made for wealth could have spread with them. He has documented that several aspects of what might now be called middle-class values changed significantly from the days of hunter gatherer societies to 1800. Work hours increased, literacy and numeracy rose, and the level of interpersonal violence dropped.
Another significant change in behavior, Dr. Clark argues, was an increase in people’s preference for saving over instant consumption, which he sees reflected in the steady decline in interest rates from 1200 to 1800.
“Thrift, prudence, negotiation and hard work were becoming values for communities that previously had been spendthrift, impulsive, violent and leisure loving,” Dr. Clark writes.
Around 1790, a steady upward trend in production efficiency first emerges in the English economy. It was this significant acceleration in the rate of productivity growth that at last made possible England’s escape from the Malthusian trap and the emergence of the Industrial Revolution. [from the NY Times review linked to above]
On a side note, something that struck me in reading the NY Times articles was that Dr. Clark seemed to be somewhat surprised that the rich had more surviving children than the poor. It seems to me that one would expect to find this in any healthy society where resources are not so plentiful that people at all levels of society can have as many children as they want. This is one of the senses in which (up until the last century) humans tend to act like the creatures they are. It's generally within the last hundred years (and at certain other points in history in various societies: the beginning of the Roman Empire and certain periods in feudal Japan spring to mind) that not having children has come to be a sign of success.
This is probably related to (though as a cause or effect I do not know) the decline in dynastic thinking in modern society. Only a very small percentage of people in our current society have family estates or businesses which have existed for many generations, and require people to think in terms of bearing, raising, educating and training the next generation in order to carry things on.
Not that people don't think about "the next generation"; but it the general attitude I hear around the corridors at work (which I take to be a more standard milieu than our friends from church or among homeschoolers) seems to be that "the next generation" is simply something that happens. One may or may not produce a few members of it, but generally it's a strange and separate entity which tends to spring up, demand video games and college tuition, and then go off to do its own thing. If people think about "carrying on the family" it generally seems to be as a somewhat backward desire usually expressed by aging parents who want grandchildren to spoil before they are too old to enjoy it.