For thousands of years, most people on earth lived in abject poverty, first as hunters and gatherers, then as peasants or laborers. But with the Industrial Revolution, some societies traded this ancient poverty for amazing affluence.

Historians and economists have long struggled to understand how this transition occurred and why it took place only in some countries. A scholar who has spent the last 20 years scanning medieval English archives has now emerged with startling answers (*) for both questions.

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The basis of Dr. Clark’s work is his recovery of data from which he can reconstruct many features of the English economy from 1200 to 1800. From this data, he shows, far more clearly than has been possible before, that the economy was locked in a Malthusian trap — each time new technology increased the efficiency of production a little, the population grew, the extra mouths ate up the surplus, and average income fell back to its former level.