China's number of newborns dropped by 320,000 or nearly 2 percent year-on-year in 2015, the National Bureau of Statistics said on January 19.

This is particularly shocking because the drop came after the government eased the one-child policy, which family planning officials said would lead to something of a baby boom. The change to the policy, which allowed couples in which either was an only child to have a second baby, came into effect at the beginning of 2014.

Yang Wenzhuang, a senior official at the National Health and Family Planning Commission, said at a forum on population issues in February last year that the change would result in over 1 million more babies being born last year. Several other commission officials echoed this prediction.

The latest figures suggest that they are terribly wrong in their projections, and the reason has to do with a failure to see what drives a population's size up or down.

A decline in newborns results from a dwindling number of women of child-bearing age and their growing reluctance to have a baby. The easing of the one-child policy did mean more couples were eligible to have a second child, but that did nothing to make women more willing to have one.

That women in the country are becoming less willing to have a child has been known for many years. Surveys conducted between 1980 and 2011 show that the average number of children that couples want to have was between 1.6 and 1.8, lower than the 2.2 needed to keep the population from shrinking.

Chinese couples actually had an average of 1.05 to 1.45 children over the past 10 years. Even in rural areas, where families are perceived to be longing for more youngsters, couples only want 1.9 on average.

As such, we have concluded that the policy change will do little to sustain the country's population because birth rates have been too low for so long. The drop in the number of newborns last year only suggests that the situation could be much worse than originally thought.

In October, the central government decided to end the one-child policy altogether, allowing couples to have a second child, apparently because the earlier easing had little effect. But given the rapid decline in the number of women of child-bearing age and their increasing unwillingness to have a baby, this latest change will not be enough to tackle an increasingly aging population and a shrinking workforce.

A national census in 2010 showed that the number of women in their prime childbearing ages, those from 22 to 29 years old, will shrink by a staggering 42 percent over the next decade.

China's population still accounts for 18.7 percent of the world's total, but it only had 12 percent of the newborns in the world last year. The country's share of the world's newborns could go down to a startling 5 percent by 2050 if the government continues imposing family planning rules.

As such, we believe that officials should quickly scrap all family-planning policies and replace them with incentives for couples to have more children. Without this, Chinese couples are unlikely to have more babies even though they can.

If the government does not act, we will see the population continue to age and the economy become stagnant in the years to come. Low birth rates will pose a big challenge soon, and our future depends on how we draw up population policies.

Huang Wenzheng is a biostatistics expert at Johns Hopkins University. Liang Jianzhang is a co-founder and chairman of Ctrip.com International Ltd.