Together, China and India currently contain nearly two out of every five people in the world — and are equal in size to the world population in 1950.

China's and India's unprecedented demographic status will not be challenged any time soon. The next five most populous countries far behind them are: the United States (318 million), Indonesia (233 million), Brazil (195 million), Pakistan (185 million) and Bangladesh (164 million).

China's and India's demographic size may also be appreciated by noting that each of their populations is larger than those of Africa, Europe or the entire Western hemisphere.

On virtually every population measure, China is further along in its demographic transition than India. With respect to mortality, for example, life expectancy at birth in China is nearly 10 years higher than in India, at 73 years versus 64 years. China's population is also much older than India's, with median ages of 34 and 25 years, respectively.

Also, while most Chinese and Indians still live in rural areas — 55% and 70%, respectively — China will soon become predominately urban, perhaps as early as 2015. In contrast, India is expected to remain mainly rural at least until mid-century.

Due to the enormous size of their populations, international migration plays a demographically negligible role in the growth of China and India. However, both have expressed official objections about illegal immigration into their countries, in particular from North Korea and Bangladesh, respectively.

In addition, the two countries have sizable numbers of their citizens living abroad for study and employment. To aid and benefit from their non-resident citizens, China and India have established governmental offices — the Overseas Chinese Affairs Office of the State Council and the Ministry of Overseas Indian Affairs.

Without a doubt, the most notable and consequential demographic trend for the growth of these two billionaire nations concerns fertility.

Although fertility levels in the mid-1950s were about the same in the two countries — at six children per woman — fertility rates have declined much faster in China than India, due in part to China's one-child family policy. Today, China's fertility is below replacement and one child less than India's — 1.8 compared to 2.8 children per woman.

With its higher birth rates and younger age structure, India's population is growing more than twice as fast as China's — 1.4% versus 0.6% annually. The demographic outcomes of these growth rates are annual additions of approximately 17 million Indians and 8 million Chinese. In addition, India's annual population increase exceeded China's in each of the past 30 years.

Both China and India have significantly more males than females, in sharp contrast to demographics in most other nations. This atypical gender imbalance is due in part to the use of prenatal ultrasound scanning to abort female fetuses.

While the practice of sex-selective abortion is prohibited in both countries, it has been difficult to prevent. By 2020, it is estimated that the number of young "surplus males" unable to find brides could be more than 35 million in China and 25 million in India.

While the future remains uncertain, it is evident that the major factor determining the future growth of the Chinese and Indian populations is fertility.

According to United Nations population projections (medium variant), Chinese and Indian fertility is expected to be at the below-replacement level of 1.85 children per woman during most of the second quarter of this century.

Based on this critical assumption, India's rapidly growing population is projected to reach 1.61 billion by mid-century, an increase of 400 million people over the next four decades.

In contrast, China's population is expected to peak around the year 2032 at 1.46 billion — an increase of 110 million — and then begin to decline slowly.

As a result, India is expected to overtake China as the most populous country in the world in less than two decades, perhaps around 2028.

Of course, other demographic outcomes are possible. One less likely, but nonetheless instructive, scenario assumes that the current fertility rates of China and India remain constant over the next four decades.

In such a case, by mid-century India's population soars to 2 billion and China's population is declining by 6 million annually, having peaked at 1.45 billion in 2030. In addition, India becomes more populous than China even earlier, in a little more than a decade.

Another intriguing scenario assumes that China changes its one-child policy to a two-child policy, with its fertility soon rising to somewhat above replacement, i.e., 2.35 children per woman, and India's fertility declines gradually to this same fertility level as China.

In this instance, the populations of both China and India would be substantially larger by mid-century, 1.62 and 1.87 billion, respectively — together gaining an additional billion people.

Here again, India becomes more populous than China within two decades. However, in contrast to the earlier scenarios, the Chinese population does not peak and decline, but continues to grow throughout the 21st century.

In addition to expected noteworthy changes in their future population sizes, both China and India will have substantially older age structures in the coming decades. Given China's older population and lower level of fertility, its future population remains much older than India's.

By mid-century, for example, one-third of China's population is projected to be aged 60 years or older. In contrast, while the proportion of elderly in India is expected to double, its mid-century level of 16% remains half of China's.

Despite the current sense of optimism due in large part to their high rates of economic growth, these billionaire nations are facing critical demographic questions. The central issue for China is whether it will soon revise its one-child policy.

Among the factors pointing to a revision of China's 30-year-old one-child policy are the growth of its economy and personal affluence, widespread national and cultural sentiments regarding children and its rapidly aging population.

However, Chinese Vice Premier Li Keqiang recently noted that China would continue to pursue a low birth rate while actively coping with problems such as sex ratio imbalance and population aging and would advance the balanced development of its population for the long term.

With regard to India, the key question is whether the country will be able to reduce its fertility level to replacement in the near future and achieve a stable population by 2045 — as envisioned in India's Eleventh Development Plan (2007-2012).

If India is unsuccessful in substantially reducing its fertility and it remains near its current level of 2.8 children per woman, the Indian population would likely reach 2 billion well before the end of the 21st century.

Officials, scholars and others often disagree on what constitutes the "best" population policies for China and India — and if adopted, whether they would be successfully implemented.

However, there's little disagreement that whatever demographic paths China and India follow in the coming decades, these outcomes have significant and long-term consequences not only for these two population titans, but also for the rest the world's inhabitants.