Even though the country`s Total Fertility Rate (TFR) has declined from about six children per woman in 1952 to about three currently, the decline has been much greater in the southern states, which have long had high rates of literacy and education. But Uttar Pradesh and Bihar, with about 188 million and 93 million people respectively, still have a TFR of about 4.3 children per woman.

During the lively online discussion, Haub said their projections assume that a smooth decline in fertility rates will, in fact, take place, reaching the two child family. "But if that does not happen, or if it takes much longer than we assumed, the resulting population would be even larger by 2101."

The factors pushing up TFR, he said, include early marriage among rural women who have little control over reproductive choices, deeply rooted traditions like son preference, which prevails even among higher income groups.

To curb population growth, Haub suggested reproductive health information and supplies be delivered to all villages, not just urban centres. Women`s education as a solution takes time, he said, but even uneducated women can be quickly informed about their choices. Men should also be convinced on the need for intelligent reproductive health choices as has been done successfully in Indonesia.

Comparing the experience of India, which is expected to overtake China`s population by 2025, and China, Haub said the latter greatly lowered its fertility rate through an involuntary programme that has been shown not to work well in India. "Coercive family planning measures would only encourage abortion of female foetuses in India," he said.

In China, both rich and poor have low fertility, he pointed out, while the problem in India is its vast rural population and its comparatively rapid growth.

With rural population outstripping available land, there will be a tide of rural to urban migration, Haub said. To stem it, he suggested creating non-agriculture employment centres as the government`s National Rural Health Mission is designed to do. Another solution is to set up more industrial estates and special economic zones.

On the other hand, Tamil Nadu and Kerala, which are already witnessing a greying of population, should address labour shortages by attracting migrants from other states, he suggested.

When a questioner argued that the ongoing economic boom in India will bring down poverty levels, improving education levels, in turn leading to a decline in population growth rates, Haub responded pessimistically, "How many people will actually benefit from the boom and to what degree? I do not see much trickle down." He did, however, see that as a concern of the current Indian government, and hopefully all future ones. Neither will the population growth adversely affect economic boom, according to him.

Haub also ruled out the possibility of Muslims ever outnumbering Hindus in the country. "I would expect the fertility rate of both groups to converge over time," he said.