But how much will scrapping the one-child limit really change the size and makeup of China's population? Not as much as you might think. There are deeper and more permanent reasons, beyond the one-child limit, that China's families have gotten so small.
One is that, for many couples, it has become very costly to have kids in China. To prepare a child to succeed in the country's competitive schools and workplaces, parents must invest lots of time and money in a child -- for schooling, extracurricular activities, and outside tutoring, often for college-entrance and English proficiency exams.
One survey conducted by the Shanghai Academy of Social Sciences in 2011 showed that families paid on average about 32,000 yuan (nearly $5,000) per year to raise a child. That may not seem like much, but in 2010, the average disposable income in Shanghai was only 31,838 yuan. According to the same 2011 survey, around 35 percent of parents in Shanghai said raising a child was a heavy burden for them, and 45.3 percent said they wouldn't have a second child, even if policy allowed.
And it's not just the money that couples are spending on kids. Hefty expenses for housing, especially in China's major cities, can eat up money that Chinese people would otherwise spend on their kids. So can taking care of aging parents. Most Chinese of child-bearing age are single kids, and they may forgo having another kid in order to better support their aging parents.
You can see evidence of these pressures in the dwindling size of families in China.
More and more couples are choosing to forgo having kids altogether, a decision that, at one point, would have almost been considered a sin under the family-centric Confucian value system. The Chinese label these young childless couples "DINK" or dingke -- standing for "dual income, no kids."
The decline in the Chinese birth rate followed a fairly predictable path. As the chart below, from Max Roser at Our World in Data, shows, population growth in a country tends to follow a set series of phases. As the country develops, child mortality drops before fertility does, and the population tends to grow.
But eventually, fertility starts to fall, and the population does, too.
This is happening in China partially because kids are healthier than ever before, and it's also because kids these days need a bigger investment from their parents in order to be competitive. When more infants survive through childhood, parents tend to want fewer children. And having fewer children means parents can devote more resources to each, according to Roser.
One study of Chinese kids recently mentioned by Tyler Cowen of Marginal Revolution supports this notion. The study looked at how Chinese twins compare to other kids, and found that the presence of an extra child significantly decreased how kids in the family performed in school, their likelihood of enrolling in college, and the health of all the kids in the family. The investment required to raise one kid into a healthy, successful adult was just so big that most parents couldn't afford to do the same for two.
The evidence in demographics
The other clue that scrapping the one-child limit may not have a huge effect is that past rule changes haven't done that much, either. While we'll never know what China's population would have looked like without the rule, many demographers believe that the effect of the one-child policy on fertility and population has been only marginal, according to Pieter Bottelier, a China expert and professor at Johns Hopkins School of Advanced International Studies.
If you look closely at China’s population, you see a curious thing: The most significant drop in China’s birth rate actually precedes the one-child policy. In 1950-1955, women in China were having more than six children on average. By 1975-1980, the birth rate had fallen to around three kids per woman. The one-child policy wasn’t enacted until 1980, after which time the fertility declined more slowly.
Even before that, there were a variety of exemptions to the policy. Ethnic minorities were exempt from the policy, and couples where both people were only children, as well as rural families that had one girl, were allowed to have two kids. Back in 2007, authorities said that the one-child policy applied to less than 40 percent of the population.
And where the policy applied, it was unevenly enforced, applying mostly to the big cities and the most densely populated rural areas. As one Chinese demographer said, the deeply unpopular policy was "more slogan than reality."
Other Chinese people developed creative ways to get around the policy -- for example, migrating to a new city. Cheng Li, the director of the China Center at the Brookings Institution in Washington, D.C., has written about how birth control was impossible to enforce among migrant workers. “My wife and I will move repeatedly until we have a male baby,” Li quotes a migrant worker as saying.
Other couples had second children, but just never reported them. An untold number of second-born Chinese were raised by their aunts, uncles or grandparents. Other wealthy people just chose to pay the hefty fine that came with having more than one child.
For these families, the freedom to have a second child without fear of reprisal will be a significant difference. And we could see a mini-baby boom as more families, especially wealthy ones, embrace this new-found freedom.
But in the longer run, China's fertility rate seems likely to stay low, and the country may even have to switch to promoting bigger families, as Singapore and South Korea have done, instead of prohibiting them. For most people, it's likely that the one-child policy has been obsolete for a while.
Xu Yangjingjing contributed to this article.