Place, Marriage, and Fertility: Evidence from a Planned City in Lowest-Low-Fertility Korea

In the study of human populations, demographers often look at two distinct moments that shape a society's future: the decision to marry and the decision to have children. For decades, the prevailing assumption in many parts of the world was that these two events were tightly linked, but in recent years, a growing body of research has shown they can be pulled apart. In some places, people marry but choose to have fewer children; in others, they delay marriage entirely, which naturally leads to fewer births. This distinction matters deeply for policymakers trying to reverse declining birth rates. If the problem is that couples are not having enough children once they are married, the solution might be better childcare or cash bonuses. But if the problem is that people are not getting married in the first place, then those same financial incentives might miss the mark entirely. Understanding which of these two forces is at play requires looking beyond national averages and examining how specific environments influence human behavior.

South Korea has become a focal point for this global concern, as it currently holds the world's lowest birth rate. Despite the government spending billions on subsidies and support programs, the number of children born per woman has continued to fall, reaching a record low of 0.8 in 2021. Yet, amidst this national decline, one city tells a different story. Sejong, a planned administrative capital built from scratch and opened in 2012, consistently reports the highest fertility rate in the country. Since 2014, its birth rate has been roughly 0.6 higher than the national average and the average for major metropolitan areas. This anomaly has sparked curiosity among researchers and officials alike. Is Sejong a miracle where families are naturally thriving, or is there a specific mechanism at work that other cities lack? A new study by researchers Changkeun Lee and Sehoon Kim sets out to solve this puzzle by peeling back the layers of Sejong's demographics to see what is actually driving the numbers.

The researchers approached the question by asking a simple but critical question: Is Sejong's high birth rate because married couples there are having more children, or is it because a much larger share of the population is getting married? To answer this, they treated the city's establishment as a natural experiment. They gathered detailed data from national censuses and birth records, comparing Sejong to a group of neighboring cities that served as a realistic baseline. These neighboring areas, including cities like Cheonan and Gongju, were the primary sources of the people who moved to Sejong, making them the perfect control group to see what would have happened if those people had stayed put. The team used a method of breaking down the total birth rate into its two main components: the rate of marriage and the rate of childbearing within marriage. They then ran simulations to see how the birth rate would change if they swapped one factor for the other. For instance, they calculated what Sejong's birth rate would look like if its residents had the same low marriage rates as the neighboring cities, but kept their own childbearing habits. Conversely, they calculated what the neighboring cities' rates would look like if they had Sejong's high marriage rates but kept their own childbearing habits.

The results of these simulations were striking and clear. The study found that the vast majority of Sejong's fertility advantage comes from the fact that more people are getting married, not from married couples having more children. In the years following the city's development, the gap in birth rates between Sejong and its neighbors was almost entirely explained by the difference in marriage rates. When the researchers simulated a scenario where the neighboring cities adopted Sejong's high marriage rates, the gap in birth rates nearly disappeared. In contrast, when they simulated a scenario where the neighbors adopted Sejong's childbearing habits, the gap barely changed. In 2015, for example, the data suggested that roughly 79 percent of the difference in birth rates was due to the higher proportion of married women in Sejong, while only about 18 percent was due to differences in how many children married couples had. This pattern held true even when the researchers looked at the data from 2020, confirming that the city's environment is primarily encouraging people to form families, rather than encouraging those families to grow larger.

To ensure these findings were not simply a result of the specific types of people who moved to Sejong, the researchers dug deeper into the demographics of the residents. Sejong is known for having a highly educated population with a large number of government employees, factors that are often associated with delayed marriage. The team used statistical models to see if these characteristics—such as having a university degree, working in the public sector, or holding a professional job—could explain why so many women in Sejong were married. They found that while these factors did have some statistical relationship with marriage, they could explain almost none of the difference. The demographic makeup of the city accounted for only about 2 percent of the marriage rate gap. This suggests that the reason people are marrying in Sejong is not because of who they are, but because of where they are. The city itself, with its specific design and institutional setup, appears to be creating conditions that make marriage more likely.

The researchers also used a sophisticated technique called the Synthetic Control Method to track the city's marriage trends over time. This method allowed them to construct a "virtual twin" of Sejong using data from other cities that closely matched Sejong's history before it was built. By comparing the real Sejong to this virtual twin after the city opened, they could see if the city's trajectory diverged from what would have been expected. The data showed that before the city opened, the marriage rates in Sejong and its virtual twin moved in lockstep. However, once the city was established and government agencies began moving in, a clear gap emerged. The real Sejong saw a sustained increase in marriage rates that the virtual twin did not experience. This divergence confirms that the change was not just a result of people moving there, but a change in behavior happening within the city itself. The environment of Sejong appears to be actively encouraging the formation of new marriages.

These findings challenge the common narrative that financial incentives for having children are the most effective way to boost birth rates. In a society where marriage and childbearing are still closely linked, as they are in South Korea, the decision to marry is the primary bottleneck. If people are not getting married, policies that offer money for having a second or third child will have little effect because those families do not exist yet. The study suggests that the unique success of Sejong may stem from its institutional design, which offers stable employment, affordable housing, and a community structure that supports family formation. The city acts as a place where the economic and social barriers to marriage are lower, allowing more people to take that step. While the researchers cannot pinpoint exactly which feature of the city is responsible—whether it is the housing policy, the job security, or the community layout—the evidence points to the environment itself as the catalyst.

The implications of this research extend far beyond one planned city. As other nations in East Asia and around the world grapple with record-low birth rates, the lesson from Sejong is that the location and design of a community matter as much as the financial incentives offered to its residents. The study indicates that creating environments that facilitate the transition into marriage may be a more powerful tool for sustaining population levels than focusing solely on childbearing within existing marriages. By treating the city as a living laboratory, the researchers have shown that place can shape fundamental life choices in ways that individual characteristics cannot explain. While Sejong is a unique case, its experience offers a compelling glimpse into how urban planning and institutional support might be used to address one of the most pressing demographic challenges of the modern era.