At the dawn of the twentieth century, life expectancy averaged 50 years in western Europe. This represented significant progress relative to previous centuries. Prior to the nineteenth century, general mortality rates were high. They also fluctuated greatly from year to year, owing to wars, epidemics, famines, and other natural or man-made disasters. In France and Sweden, for example—two countries for which long-term data is available—life expectancy at birth varied between 20 and 40 years (see Figure 1). Toward the end of the eighteenth century, average mortality rates began to rise, then from the mid-nineteenth century onward, the significant fluctuations of the previous period began to subside—both in France and in Sweden, and almost certainly in other countries experiencing early industrialization too, such as England and Wales and Switzerland. These changes saw life expectancy reach 40–45 years in the first half of the nineteenth century, and 45–50 by around 1900. In France, a man living his entire life in the sanitary conditions of the year 1900 would have had only a one in three chance of surviving to the age of 65. For a woman, this was slightly higher, at 40%. A century later, in 2000, these figures had risen to 80% and 90% respectively, with life expectancy at birth reaching around 80 years (slightly more for women and slightly less for men). With the exception of two periods of increased mortality corresponding to the two world wars, life expectancy has progressed steadily throughout the twentieth century in Western countries, although the factors that explain this impressive development have changed over time…