(CNN) In October 2015, China decided to overturn its decades-old one-child policy and allow couples across the country to have two children.

The change was much anticipated by demographers worldwide, who would have preferred scrapping the policy altogether, considering it an injustice of human rights that changed the population structure for the worse. Researchers explored the potential effect in a new study , published Thursday.

Almost a year after the decision, the study warns that the Chinese government's goal of a population increase may take more than two decades to arrive.

"It's something we've all been waiting for, for a long time," says Therese Hesketh , professor of global health at University College London, who led the research. "But the effects are only going to come through in the adult population."

In other words, at least 20 years away.

No boom in sight

The paper predicts that the updated policy will bring a peak in population size of 1.45 billion people in 2029, compared with 1.4 billion in 2023 if the one-child policy had stayed in place.

The one-child policy was introduced in 1979 when the government feared a rapid increase in population size after the baby boom of the 1950s and 1960s. The country's fertility rate fell dramatically, from a peak of almost six births per female between 1960 and 1965 to 1.5 per woman between 1995 and 2014.

In the years in between, amendments and caveats were added, allowing rural couples to have a second child if their first was a girl and then allowing couples who are only children to have a second child.

In the time it has taken to overturn the ruling, many other factors about China's population and economy have changed, most notably its level of urbanization. As people go to increasingly larger cities in search of employment and income, their desire to have large numbers of children reduces, Hesketh pointed out, adding that a report of hers found that "90% of Chinese people only want one or two children."

Problems will persist

The paper further highlights that problems stemming from China's uneven population structure will continue to exist. There will still be a need for an increased labor force as the nation's elderly population increases rapidly and the country has more young males than females, in part because sons are more desired and considered better able to work and support the family.

"There were considerably more males than females in the country being born," Hesketh said. "(And) the number of elderly people is a major concern."

In 2015, 9.55% of the population in China was over the age of 65, compared with just 3.36% in 1965. China is home to 114 million people over this age, and up to 90 million people are expected to retire in the next three decades, leaving the labor force with even fewer people until this year's newborns are old enough to work.

By 2050, 29% of the population would have been older than 65, compared with 25% under the universal two-child policy, according to the paper. The labor force would be predicted to fall to under 750 million by 2050, compared with approximately 800 million under the two-child policy.

Then there's the problem of too many men.

At birth, there have been 1.15 males to every female born in China in 2016, the most skewed gender ratio in the world. The paper predicts that almost 30 years of the one-child policy will, by 2020, leave an excess of 30 million men who are unlikely to marry and that this ratio will take time to even out.

"We don't expect the sex ratio to go back to normal that quickly," Hesketh said, adding that a surprising benefit of the inequality in numbers has been to females, and it has helped empower them. "I hear people say this is because resources went to girls in a one-child family."

There are some likely positives from the two-child policy as well, as the researchers predict that certain societal issues that arose from the original policy, such as unregistered children and abortions of unapproved pregnancies, would fall significantly.

But the biggest challenge remains: the elderly.

Looking after the current population

To cushion the consequences of this lag in population increase, Hesketh has a series of recommendations to support the aging population. The first is to increase the national retirement age: China has one of the lowest in the world, at 55 for women and 60 for men. Next comes the strengthening of state pensions, particularly for rural pensioners who are more likely to be living alone while their child lives in the city, and to increase options for three-generational living, in which parents live with or near their children in the city.

Three-generational living would require both policies and subsidies, Hesketh highlights, because as with urban life in any other country, the cost of living is high.

Three-generational living "would be a controversial proposal," said Robert Pozen , a senior lecturer at the MIT Sloan School of Management, who specializes in this field and was not involved with the paper.

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"It depends on what part of China you're talking about ... and the level of subsidy," he says. Pozen believes the reality will involve a dual approach with families encouraged to live as one but also for China to build more nursing homes and houses with assisted-living facilities.

"You already find more and more elderly people who are not looked after by their children or grandchildren," Pozen said. "The support of children has already declined."

But for the rest of the recommendations, he agrees wholeheartedly.

"We need to push the pension system in China to rural areas. We have already, but it is modest," he said, mentioning the "practical problem of collecting social security payment."

He added that a revised retirement age is also warranted "But that will have to be brought in gradually," he said. "If it's done too quickly, there will be protest ... but if it has to be gradual, it will not have a big effect immediately."