Between 1800 and 1850 the human experience began to change. Before 1800, life was short and few made it into what we would now consider old age. Most women spent much of their life having and caring for children. The total human population was small, and with high mortality rates and low life expectancy, population growth was slow. After around 1850, major changes occurred, at first slowly and only in the richer countries, then more quickly as the transition diffused across the globe. Mortality rates declined, women had fewer children, people lived longer and the population increased.

Three basic statistics make the point. In 1700, the global average life expectancy was only 27 years of age; the global average fertility rate was six children per woman and only 4 per cent of the world's population was aged over 65. By 2020, the average life expectancy was 72 years of age, the average fertility rate was 2.4 and 9 per cent of the population was aged over 65. People lived longer, fertility rates fell and the population aged. This change is referred to as the ‘demographic transition’ or sometimes as the DT model. Remember, it is a model that simplifies the complex reality of the world. Demographic patterns before 1800 were more complex than the model posits. There were periods of rapid growth. In England, in the eighteenth century, as Tony Wrigley and colleagues have shown, there was a marked increase in marriage fertility and a consequent increase in population growth and in total population (Wrigley & Schofield 1989; Wrigley et al. 1997). War and famine reduced the population while peace and prosperity allowed population levels to rebound and grow.

There were also variations over space. Studies of both Belgium and Spain, for example, uncovered different fertility levels by region in the past and more recently. The differences are in part a result of varying levels of secularization, which is a reminder that variations in local cultures can produce marked differences in fertility levels (Lesthaeghe 1977; Lesthaeghe & Lopez- Gay 2013).

There are two important features to point out. First, there were and still are vast social differences behind global and even national averages. The rich tend to live longer than the poor.