One in three people worldwide lives in either China, the largest communist country, or India, the largest democracy.

For the moment, China remains the most populous nation, with 1.3 billion inhabitants, followed by India, which is home to 1.1 billion.

But India's higher fertility rate means the gap is narrowing and the UN expects it to overtake China before 2030.

Both countries are also experiencing rapid growth in their urban populations.

In China, the number of people in towns and cities is likely to exceed the number in the countryside by 2015.