New crops and population growth

Why did one new crop change a continent's population more than a century of good harvests?

Europe had good decades before the potato. Warm summers, full barns, cheap bread — and populations that rose during them and subsided again, leaving the long trend where it was. Then a South American tuber spreads across the north, and the curve bends and stays bent.

Why should one crop do what a century of good weather could not? And — the harder question — how would we know the crop did it at all, rather than a growing population reaching for whatever fed it best?

Start with the difference between a good year and a new crop, because it is the whole mechanism. A good harvest raises the food available this year. In a pre-industrial economy that surplus does not sit idle: mortality falls a little, marriages happen earlier, more children survive. Population absorbs the surplus, and when the weather returns to normal the extra people are still there, pressing back down on the standard of living. A run of good years raises the level temporarily and leaves the ceiling untouched.

A new staple does something categorically different. It raises the calories a given hectare can yield, permanently. The same land now supports more people at the same standard, and the population climbs until it meets the new ceiling. One is a transient shock to the level; the other is a shift in the constraint.

So why this tuber? Several properties compound. It yields substantially more calories per hectare than the rye or oats it competed with in northern Europe — commonly given as roughly two to three times, though published figures move a good deal with soil, variety and accounting method, so treat the multiple as an order rather than a value. It grows in cool, wet, poor ground where cereals do badly, it sits below the surface where hail, wind and passing armies reach it less, and it could go into ground that would otherwise have lain fallow, adding a crop rather than displacing one.

Now the hard part. The correlation is not in dispute: potato-growing regions grew. The direction is. A population under land pressure has every reason to adopt whatever feeds most people per hectare, so adoption could be the consequence of growth. Worse, both could follow something else: the retreat of plague, the end of the coldest phase of the Little Ice Age, stronger states, other American crops arriving on the same ships.

What would break the loop? Something that varies across places, is plainly not caused by their populations, and predicts how much potato they could grow. Nunn and Qian's study uses exactly that: agronomic suitability of land for potatoes — a function of soil and climate, fixed long before anyone lived there — interacted with the period after the crop's introduction. Well-suited and badly-suited regions are compared before and after, so the estimate rests on the change in the gap between them rather than on any cross-sectional comparison. Their headline result attributes roughly a quarter of the growth in Old World population and urbanisation between 1700 and 1900 to the potato (recalled).

The design is strong but not an experiment, and its limits should be said plainly. It is a difference-in-differences on an interaction term: any other post-1700 change favouring cool, wet, potato-suited land loads onto the same coefficient. Pre-1800 population figures are reconstructions with wide uncertainty, suitability is computed from modern varieties and climate, and one continental average is asked to stand for places with very different institutions.

Here is the falsification test, and the record can answer it. If the mechanism is a raised ceiling in a population-responsive economy, the gain should appear as more people at roughly the same living standard, not as durably higher wages. Had potato-suited regions instead shown lasting real-wage gains without a population rise, the ceiling story would be wrong. The broad picture of European real wages across the potato's diffusion is not one of durable improvement, which is what this account predicts.

Two refinements connect the steps. First, "the potato caused growth" is shorthand for something narrower: it raised the sustainable population of land that suited it, in an economy where births and deaths responded to living standards. Where that responsiveness is absent — as in an industrial economy — the same calorie gain shows up as income rather than as people, which is why the argument does not transfer forward in time. Second, the crop did not act alone: maize spread through southern Europe, Africa and China over the same centuries, and the potato's diffusion depended on institutions and diets that would tolerate it, which is why adoption lagged introduction by over a century in places.

The account also carries its own dark confirmation. A population that has climbed to a level supported by one crop is exposed precisely to that crop's failure. Ireland after 1845 is the case: conventionally about a million deaths and a comparable number emigrating (recalled, and the estimates are genuinely contested). That episode does not show the potato was a mistake — it shows the mechanism worked, because the people were there only because the ceiling had risen.

• Why maize's spread through Africa and China is harder to identify than the potato's.
• Neighbours set the boundaries: Domestication explains how a wild plant becomes a crop, and Monoculture fragility what happens when a population rests on one genotype; this piece asks only whether a new crop moves a population, and how anyone could tell.

Every term the collection defines is gathered in the glossary.