With 1.4 billion and 1.2 billion people, China and India account for 37 percent of the world population today. Thirty years later, they are expected to account for roughly the same percentage of the world population. The overall numbers, however, hide some fundamental changes that would have occurred by then.

In another 20 years, India is expected to catch up with China in terms of population. The UN projections of population indicate that in 2030 the two countries are expected to have about 1.4-1.5 billion people each, with India marginally ahead of China. The period between 2025 and 2030 will be significant in terms of China's demography, with the rate of population growth reducing to zero during that time.

By then, China would no longer be making net addition to its population. The number of births will match the number of deaths, with the latter gradually overtaking the former to push China into a negative rate of population growth after 2030. This will be the culmination of a remarkable demographic transition in China. It will mark the end of a period comprising almost seven decades during which China would have reduced its rate of population growth from 2.6 percent (mid-1960s) to below zero.

India's population, on the other hand, will continue to grow. At the time when China is expected to have negative population growth, India's population is projected to be growing by about 0.6 percent a year. Indeed, India is not projected to reach a zero rate of growth in population for more than two decades after China does. It will have about 1.6 billion people in 2040 compared with China's 1.4 billion. The combined China-India population is expected to increase from the present 2.6 billion to 3 billion by 2040, with most of the increase coming from India.

These big demographic changes will be accompanied by more subtle demographic transformations. The most discussed and familiar of these is the change in the relative weights of the working-age population (15-60 years) in the total population.