From The New York Review of Books:
A Farewell to Alms: A Brief Economic History of the World (Princeton Economic History of the Western World) by Gregory Clark:
In any society at any time there is a reasonably well defined notion of “subsistence,” a level of income, essentially wages, just adequate to support a standard of living that will lead the average family to reproduce itself. Subsistence has a hard physiological basis in calories, necessary nutrients, protection from weather, and the like, but it can be modified by cultural factors, social norms, and customs. If wages happen to exceed subsistence for a while, because of good harvests or a reduction in the supply of labor through war or disease, normal mortality will decrease, fertility may rise, and the population will increase. But not for long: the pressure of a larger working population on a fixed supply of land and resources will force labor productivity and wages to fall. (That is the famous law of diminishing returns: the idea is that as more and more workers are squeezed onto the same area of land, at some point each additional worker will be able to add less output than his predecessor did, simply because he has a smaller share of the land to work with.)
This process cannot stop until wages are back to the subsistence level. The population will be bigger, but its members no better off than they were. If harvests then go back to normal, productivity and wages will fall below subsistence and the process just described will go into reverse: higher mortality will cause population to fall until productivity and wages return to the subsistence level and then stabilize. This is a simple and powerful story, and it has just the implications needed to explain the grim preindustrial history. The key implication is that the material standard of living of any population is determined only by the level of subsistence. Incremental technological progress, which certainly took place in England — and elsewhere — between 1200 and 1780, does not seriously improve living standards; it just allows a larger population to be supported.
More here.