The First Population Bomb

How Structural Change Drove Population Growth in Early Modern Britain

First came the deluge. Though British gross domestic product rose swiftly during the late eighteenth century, it had nothing to do with higher living standards, as income per capita was stagnant for the first decades of the Industrial Revolution. Instead, the population exploded. From 1740 to 1840, the number of Britons tripled, rising from little over 5 million to just less than 15 million. Apart from settler colonies, which coupled high birth rates with massive immigration, such a surge was unprecedented in the history of humankind. No pre-industrial economy had ever experienced such dramatic growth over such a prolonged period, and the greatest testament to British industrialization is the mere fact that it didn’t collapse outright under the pressure of so many extra mouths. But why did Britain’s population start expanding so rapidly—and why did that spurt precede the economic changes that followed?

During the early eighteenth century, little sign of the demographic explosion could be found. The intrinsic growth rate (IGR)—the rate at which population will rise or fall if present fertility and mortality patterns persist—had been essentially zero during the second half of the seventeenth century and increased only marginally (to 0.2 percent) up to 1741. Indeed, the late 1720s were wracked by mortality and famine, cutting the population by 200,000, and there’s evidence to suggest that even by mid-century, Britain had not yet reached the maximum total of six million achieved before the Black Death. This latter fact is telling: low population was due not to poverty, but the underlying demographic regime. Prior to the eighteenth century, the West European Marriage Pattern ruled in Britain (as well as France, the Low Countries, and Scandinavia). New couples were, by cultural convention, expected to set up their own independent households, creating an economic barrier delaying marriage. The duration over which women gave birth, therefore, was governed not by biological possibilities, but by the time it took for a couple to acquire adequate funds to start a new household. Women married in their late twenties, not as teenagers, reducing family sizes by 2.8 children as some of mothers’ most fertile years were spent out of wedlock. Many never married, as they failed to make the money needed to set out on their own.

As a result, population growth tended to be moderately self-regulating. Though total numbers did increase slowly in the long run, short-term accelerations reduced real wages and consequently delayed the date of marriage for many couples. When population growth peaked during the seventeenth century and incomes declined, so did fertility, by as much as 29 percent. This was the Malthusian “preventative check” in action, and it contributed to a low-pressure demographic regime. Population never grew very fast, but on the other hand, real wages never fell very far, either. Amid the economic hardships of the late seventeenth century, as noted above, growth was mildly negative. From then on, however, IGR was positive and accelerating, averaging 1 percent from 1740 to 1840 and peaking at 1.75 percent in 1821. Population growth can increase because fertility rises and because mortality falls; in Britain, both happened. Between the seventeenth-century zero-growth equilibrium and the nineteenth-century liftoff, the gross reproduction rate (GRR) rose from 1.95 to 2.66 births per female, while life expectancy rose from 34 to 41 years—accounting for 64 and 36 percent of the increase respectively.

But why did these two variables change? What broke Britain out of stagnation? We’ll take on fertility first. The GRR can rise for multiple reasons: higher within-marriage fertility rates for women; higher fertility among the unmarried; earlier marriage by women; or a fall in the proportion of single women. All four changed in Britain prior to the nineteenth century. Marital fertility rose, especially within the 15-19 age group, thanks to a sharp increase in the proportion of women pregnant on marriage—in the early nineteenth century, fully one-quarter of babies were prenuptially conceived, up from 7 percent a century before. This was boosted by a 5 percent decrease in the interval between births, driven by a collapse in the rate of stillbirths (from 100-125 per thousand to 40-50). This alone accounted for 7 percent of the fertility jump. This is somewhat strange. Stillbirth rates are determined primarily by infant weights—the rate at 2,500 grams is 10 to 30 times that at the optimal weight of 3,500 grams—which are in turns functions of maternal nutrition. Yet by most accounts, calorie availability in Britain was falling during this time of dramatic change. The proportion of illegitimate births also rose (from 1.8 to 6.2 percent), adding another 4.7 percent to fertility.

But nuptiality trends—the proportions and ages of women marrying—were the most significant factors in the fertility shock. They could be extremely potent; small changes to either variable could least to an elastic response in births. A one-year decline in mean age at marriage, from 26 to 25, could increase the average number of children born to each woman by 7 percent. Male and female marriage ages tumbled across the “long eighteenth century,” by 2.5 years in both cases, which corresponded to at least a 20 percent rise in marital fertility. Nuptials increasingly became the province of the young; the proportion of weddings joining men aged 20-24 with women of the same or lesser years doubled from 22 to 41 percent. Combined, these patterns increased GRR by 28 percent, accounting for three-quarters of the total rise observed over the period. This leaves the proportion never married, which did not change much during the eighteenth century. Yet this variable was sensitive to real wages and the success of harvests; in line the with the Malthusian “preventative check,” more marriages occurred in good years than bad. And the late eighteenth century saw a significant reduction in the number of crop failures. To this is attributed the small residual left over after within-marriage fertility, illegitimacy, and age of marriage are taken into account.

Scholars once thought that reduced mortality had been the primary force behind Britain’s demographic boom, but the work of historians like E. A. Wrigley has since dispelled such notions. Nevertheless, there was advance on this front, too. Adult mortality improved across all age brackets, implying a ten- to twenty-year increase in life expectancy at birth. Childhood mortality, however, was largely stagnant, and the only real change was a drastic fall in the number of deaths in the first month of life. Another way to look at this: of 1,000 infants surviving the first month of life in 1650, 761 would still be living on their fifteenth birthday, a figure essentially unchanged at 778, in 1800. But for every 1,000 adults living on their 25th birthday, only 419 would still be living at age 60 in 1650, whereas around 1800 the comparable figure was fully a third higher, at 561. Wrigley suggests that this marks the emergence of “a ‘modern’ pattern in age-specific mortality which had been conspicuously absent at the start of the period.”

Lower urban death rates played a major role. Pre-industrial cities were “demographic sinks,” requiring immigration from the more salubrious countryside to sustain or increase their populations in the face of high mortality. As late as 1861, London’s life expectancy was eight years lower than that prevailing in rural areas. In the late eighteenth century, however, first London and then several other of the provincial cities began to self-replace their unhealthy denizens. But the overall effect was mitigated by British structural change; since the agricultural share of the population was decreasing, more people came to live in higher-mortality areas of Britain, producing a negative composition effect on overall mortality rates, especially during the early nineteenth century. Another outstanding factor was the fall in maternal mortality, which collapsed from 16.8 to 5.3 deaths per thousand births between the late seventeenth and early nineteenth centuries. This, interestingly, was a pan-European phenomenon, present in both Sweden and France at the same time. These factors are difficult enough to explain, given evidence of declining calorie availability and adult stature, that a recent paper by Crafts and Mills (2021) argues unhesitatingly for interpreting the mortality changes as exogenous shocks.

But these are all proximate causes of Britain’s population surge. Why, exactly, was the classical demographic regime prevailing until the mid-eighteenth century superseded by an efflorescence of exponential growth? In one sense, it wasn’t. Instead, the material factors that governed population under the classical Western European Marriage Pattern changed, and thus elicited a strong fertility response. The Pattern tied fertility to material well-being: nuptial age and the proportion of celibates were governed by real wages and the success of harvests, creating a self-regulating mechanism that slowed population growth in hard times and boosted it in plenty. The rise in population from 3.1 to 5.1 million from 1551 to 1636 was accompanied by declining incomes and increases in both the age at marriage and the fraction never marrying. Harvest failures during the late sixteenth century produced classic Malthusian positive checks, with death tolls surging during the famines of the 1590s.

But in Britain, times were getting consistently better. Good fortune and high agricultural productivity—a result of the country’s precocious structural change—limited the extent and number of food crises. While in 1693-4 a bad harvest in French Beauvaisis caused an increase in grain prices and severe mortality, Britain absorbed the shock with only a minor dent to real wages. This index climbed steadily thereafter, and age at first marriage moved in symmetrically the opposite direction. Moreover, the movement of cultivators from farm to town and rural industry changed both the material and social inhibitions that upheld the self-regulating British system. Fewer farmers meant fewer live-in servants who—forced to remain single—had to complete enough years of work to attain the earnings required to marry. More young urban dwellers earning wages meant an enlarged group of fertile individuals with the wherewithal and independence from rural social norms to form earlier couples. This latter factor also contributed to the sharp rise in illegitimacy. The highest crude birth rates, unsurprisingly, were to be found in the industrial counties and cities. Movement to cities also propped up agricultural wages, allowing more couples to form by the traditional means and thus pulling down the age at first marriage across the board.

Britain’s urbanization, real wages, and population growth were the three factors that distinguished her from the other pre-industrial economies of Western Europe. While the Western European Marriage Pattern was prevalent on the continent, only in Britain did the material and social conditions arise to turn that demographic regime’s self-regulatory mechanism into a fertility surge. Urbanization—sparked by the exogenous impulse of trade—broke the real wage-fertility cycle of the early modern period and increased the pressure of the demographic regime by changing economic conditions and social norms. But the road from structural change to population growth was not a one-way street; larger populations could in turn impact positively on the rate of economic growth, creating another positive feedback mechanism in the critical early years of the Industrial Revolution. I’ll look at this reverse channel in a subsequent post.