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Growing and Shrinking

Eros, Thanatos, and the demographic balance in the ancient world

From biological-instinctual conditions to individual choice

The world's changeable geodemography

Revolution and demographic transition: From 1 to 10 billion in two centuries

Men and women of the Homo sapiens species have not long been on this Earth: only for some 100,000 years. In the long arc of the world's biological evolution, this is a very short time indeed. Like other animal species, humans are motivated by a strong survival instinct. The concept of instinct is controversial and lends itself to various different interpretations. In more simple terms, we could define it as the constant attempt to avoid suffering and put off the unpleasant event that is death. The reproductive instinct is helpful to us in this sense, because the existence of family descent and the solidarity between parents and their offspring strengthen our capacity to survive. Children unable to take care of themselves survive thanks to parental care, just as old people unable to take care of themselves survive thanks to the care their children provide. The reproductive instinct and the survival instinct - Eros and Thanatos - have the same matrix and are both sustained and strengthened by experience: death is painful, reproduction is pleasurable. Over a very long period - centuries and millennia - these two instincts remained in a certain equilibrium, so to speak. Excessive reproduction (too many offspring) weakened parents' capacity to provide adequate care, protection, and nourishment to each child, thus making their survival more precarious, while a reproduction deficit (too few offspring) brought the extinction of the family, group, or clan. The balance between reproductivity and survival was continually shaken, and indeed tested, by conditions determined by external factors like climate, environment, diseases, and the availability of energy resources and food. These factors are not fixed: they change due to both natural forces (climate cycles, the biological evolution of diseases) and human ones (penetrating into new environments and new spaces, or producing new resources). Over time survival and reproduction fluctuated in tandem with these factors, but within rather restricted limits. Indeed, humans have proven able to elaborate an infinite number of strategies consistent with their survival instinct, as they sought to minimise and reduce the risks of dying and the suffering connected with it. For a long time - until the dawn of modernity - these strategies were of modest success to none: the historical and prehistoric evidence tells us that there were no significant variations in mortality and longevity. People died with more or less the same frequency in the Sun King's France, in Elizabethan England, or in the Venice of the doges as in the age of Augustus. The construction of complex and refined forms of society, the elaboration of philosophical thought, the evolution of scientific thought, and the fundamental discoveries of mathematics, physics, or astronomy helped little. Even where some people did manage to overcome the material dangers of poverty and hunger - as in the case of privileged groups of aristocrats, merchants, and landowners - mortality remained just as high among the poor layers of the population - serfs, commoners, and labourers. The destructive power of diseases, particularly infectious and transmissible ones, remained unabated. 'Invisible enemies' - microbes - were known to exist but could not yet be observed. They took charge of levelling out mortality among different social groups and layers. Even the instinct of reproduction remained circumscribed within rather limited boundaries: demographic and historical research has demonstrated that up to the end of the eighteenth century couples' sexual behaviour was largely if not totally lacking in deliberate regulation, being conditioned instead by biological or biology-related factors such as age, one's state of health, the frequency of sexual relations, and the length of the nursing period.

This premise helps us understand why across the history of humanity demographic growth has overall been modest and for long periods almost imperceptible. Biology and external constraints conditioned the two interdependent mechanisms of growth: survival (mortality) and reproduction (fertility). The poverty syndrome that gripped humanity - poverty in material resources and, above all, in understanding - kept mortality high. The average (mean) lifespan (or life expectancy at birth) rarely exceeded a third of a century. It was difficult to defeat the invisible enemy, the microbes that generated the most lethal diseases striking out of nowhere. Reproduction levels were kept high, not only in order to compensate for high mortality rates but also because humans had not 'discovered' how to limit the number of births - on average, no less than five or six per woman.1 Naturally women and men discovered soon enough that the sex act led to childbirth, but they were not culturally equipped to separate the one from the other. Thus in the long term births and deaths remained in rough equilibrium, if we look past the - sometimes even violent - fluctuations generated by external factors and unpredictable shocks.

If we accept the plausible hypothesis that 10,000 years ago - in the era of the first beginnings of agriculture - the Earth counted a few million human inhabitants (let's say 10 million) and that in the era of Christ - and here we are on firmer ground - it counted 250 million, arithmetic tells us that the mean increase was around four extra individuals per 10,000 people per year. This was an imperceptible rate of increase. A higher mean rate of increase allowed the quarter-billion of year 0 to reach 1 billion in number around 1800, but there was still very little growth: around eight extra individuals per 10,000 people per year. These very modest growth rates, calculated across millennia, are abstract representations of demographic evolution that did not proceed at any uniform pace but through fluctuations and cycles, highs and lows, increases and decreases. Nonetheless, these rates do express the fundamental potential for population growth, which, in the long run, was rather modest.

We can identify with a degree of certainty some of the important phases that saw changes in the pace of population growth, for example after innovations that produced leaps forward in resource availability. First of all, the transition from a system of obtaining resources based on hunting and gathering to one based on agriculture, which made it possible to expand, regularise, and maintain food production and encouraged populations to settle, thus freeing them of the hardships and risks of nomadism. Secondly, the agricultural and industrial revolutions, which brought a strong expansion of food and energy resources from the eighteenth century onwards. Thirdly, the opening up of new spaces for settlement, as happened with the populations that came out of Africa in distant times and with the migration towards deserted or little populated regions and continents, in search of better living conditions. Not all these changes of pace produced acceleration, and others slowed or reversed population growth: for example the combined development of diseases, populations' vulnerability and immunity to them, and the emergence and spread of new diseases. Such was the case of the European plagues in the Justinian and late medieval cycles, or smallpox in the Americas.

The balance between survival and reproductivity that had lasted for millennia began to waver in the European and European-origin countries of the eighteenth century. This period saw a shattering of the syndrome of both material poverty and poverty of knowledge, which had acted as a powerful dam against demographic change, keeping both mortality and fertility at high levels. The agricultural and industrial revolutions increased the resources available to individuals, and this meant more energy, more resources in raw materials, and thus more manufactured goods and more food. Economists use the word 'development' to characterise this process, which came about in a complex manner. Scientific innovations in the biological and medical field identified the causes of the more common transmissible diseases and the ways to avoid or prevent them. Survival rates and lifespans increased. The survival instinct thus came to operate in a profoundly different context; yet, if it were possible to measure this instinct, we would probably find that its strength had still remained undimmed. Even if this instinct had been weakened, people's survival capacity increased. Some diseases were eliminated by public healthcare and others were taken care of by medicine, even among those individuals who had lost the will to live. Thus the survival instinct was no longer the dominant factor in the eternal effort to put off death and the suffering connected with it.

The reproductive instinct also ceased to be the decisive factor in generating offspring. Children's survival rates increased and couples were driven to have fewer of them, by adopting a practice they always knew of but had never - or only sporadically - practiced: voluntary birth control. The bind between sexuality and reproduction was cut. Parents were no longer the only source of investment in children; social mechanisms for providing education, healthcare, and protection made...