The world population reached the first billion mark in 1820. And after that it started growing at a faster and faster pace, firstly, due to a greater decline in the death rate in the developed countries and then due to a decline in the death rate in the developing countries.

Table 5.2 shows the various dates when the world population reached various billion marks. It exhibits that in the early nineteenth century, the population started growing. Yet, the growth rate was low. As explained above, the reason is that this growth was confined to the developed countries only which comprised one fifth of the world population. In the developing countries life expectancy at birth was around 20 and death rate was around 50. Thus, despite a high fertility, the population was constant. World population started growing fast in the twentieth century, particularly in the second half of the twentieth century. This is the time when mortality rates declined with suddenness in the less developed countries and their fertility rates remained unchanged.

If we assume that it would take nearly 12 years again to reach the next billion, we can roughly say that some time in the year 2011, the world population will reach the 7 billion mark. Thus in the period between 1820 to 2011 (less than two hundred years), the world population has grown nearly seven times. Nobody in the eighteenth century could have imagined this explosion of population.