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By Root 541 0

produce food for sale not for self-sufficiency. The division of labour increases. But when the food supply becomes tight, near the top of the graph, fewer people will be prepared to sell their food or will have a surplus to sell. They will feed it to their families and make do without the goods they were wont to buy from others. The non-farmers, finding both food and customers for their services harder to come by, will have to give up their jobs and return to growing their own food themselves. So there is a cycle of rising and falling specialisation in human populations. The economist Vernon Smith, in his memoirs, recalls how in the Depression his family moved in the 1930s from Wichita, Kansas, to a farm when his father was laid off as a machinist, because ‘we could at least grow most of our own food and participate in a subsistence economy.’ This return to subsistence happens often in human history.

In the animal world, this is unique. In no animal species do individuals become more specialised as population is rising, nor less specialised as population is stalling or falling. In fact, the whole notion of specialised individuals is rare outside the human race, and where specialisation does happen – in ants, for example – it does not wax and wane in this way.

This suggests that good old-fashioned Malthusian population limitation does not really apply to human beings, because of their habit of exchange and specialisation. That is to say, instead of dying from famine and pestilence when too numerous for their food supply, people can increase their specialisation, which allows more to subsist on the available resources. On the other hand, if exchange becomes harder, they will reduce their specialisation, which can lead to a population crisis even without an increase in population. The Malthusian crisis comes not as a result of population growth directly, but because of decreasing specialisation. Increasing self-sufficiency is the very signature of a civilisation under stress, the definition of a falling standard of living. Until 1800 this was how every economic boom ended: with a partial return to self-sufficiency driven by predation by elites, or diminishing returns from agriculture. It is hard to be sure given the patchy information that this is what happened to Mesopotamia and Egypt after 1500 BC, or India and Rome after AD 500, but it is pretty clear that it happened to China and to Japan in later centuries. As Greg Clark puts it, ‘In the preindustrial world, sporadic technological advance produced people, not wealth.’

The medieval collapse

Robert Malthus and David Ricardo, though they were good friends, disagreed on much. But in one respect they were entirely aligned – that unchecked population could drive down the standard of living.

Malthus: ‘In some countries, the population appears to have been forced, that is, the people have been habituated by degrees to live upon the smallest possible quantity of food ... China seems to answer this description.’

Ricardo: ‘The land being limited in quantity, and differing in quality, with every increased portion of capital employed on it there will be a decreased rate of production.’

At first glance, medieval England furnishes a tidy example of such diminishing returns. The thirteenth century, a time of mild weather across Europe, saw a prolonged expansion of the population, which then crashed in the following century as the weather deteriorated. The 1200s were the golden high-water mark of the Middle Ages. Courts were richly furnished; monasteries flourished; cathedrals rose towards the sky; troubadours strutted their stuff. Watermills, windmills, bridges and ports were built all over England. Fairs and markets proliferated and thrived: there was an unprecedented surge in commercial activity between 1150 and 1300. A good part of it was driven by the wool trade. As Flemish merchants sought out more and more English wool to supply the cloth makers of Flanders, so they provided livelihoods for ship owners, fullers and above all sheep farmers. The national sheep flock boomed