The United States of America has been urbanizing since the first European settlements. Urbanization accelerated between 1830 and 1930, a period during which the U.S. went from being 9% to 56% urbanized. After more rapid growth from 1940 to 1970, urbanization slowed over the next four decades, stabilizing at 81% as of 2010 ^[1][29]. In 2022, the United States’ urbanization rate was around 83% ^[2][30]. Research shows that the oldest and largest metropolitan areas in the U.S. are seeing negative net migration and are losing population. By contrast, growth is occurring in the “established midsize, interior cities through all regions of the country” ^[3][5]. While there is a steady stream of people migrating out of core cities and into U.S. suburbs, Golding and Winkler ^[4][31] caution against declaring the end of urbanization in the U.S. because growth from rural migration is occurring in “exurbs” just outside metropolitan areas. They argue that the nation is still experiencing urbanization but as a phenomenon extending beyond urban cores. The suburb growth rate in the 1950s was 50%, and over 10% in the 1990s, but the share of the metro population living in the core city went from 60% in the 1940’s to less than 40% in the 1990s ^[1][29]. Urbanization and economic policies are intertwined, and much of developing countries’ urban acceleration can be explained by new economic systems or government policies and initiatives ^[5][28]. China’s economic transformation was the main contributor to the surge in internal migration, with an increasing percentage of jobs created by state, private, and foreign entities. With the normalization of relations between the United States and China following Nixon’s visit to China in 1972, Carter’s reopening of diplomatic relations in 1979, and the entrance of China into the World Trade Organization in 2001, U.S. foreign investment mushroomed, and China became the factory of the world for over a decade. Low labor costs and fewer environmental regulations meant cheap products, savings for U.S. consumers, and larger profits for companies and shareholders ^[6][32]. At the same time, jobs in manufacturing cities in the U.S. declined. Once scarce, urban jobs in China became abundant as it further industrialized to meet U.S. and global demand ^[7][33]. In recent decades, China’s urban residential migration has become less government-planned and more market-oriented. These driving factors created China’s explosion in internal migration. These relocations resulted in the nation going from a 20% rate of urbanization in 1978 to becoming an officially urban nation in 2011, with 56% of its residents living in urban areas and around 65% in 2022 ^[8][34]. Implemented in the 1950s, China’s household registration system (hukou) linked citizens to services based on their housing location and classified them as either agricultural or non-agricultural (urban). Those with an urban designation had access to jobs in cities and received substantially more state-sanctioned benefits, including food and housing. To a large extent, the hukou determined the receipt of benefits and the ability to get an education. A change in hukou registration needed government approval, which was hard to obtain. In 1978, reform to China’s hukou system, which historically has regulated migration and restricted mobility ^{[9][10][11][12][13]}[11,35,36,37,38], partly opened the door for the most rapid urbanization in history, with massive amounts of rural workers migrating to urban areas ^[11][14][10,36]. There have been multiple phases of hukou reform since. While food supply and employment opportunities have not been hukou-based for a few decades, welfare services and benefits are still attached to hukous. These urban benefits center most around education, healthcare, and social security ^[7][33]. Only in 2003 were police banned from stopping and deporting people from cities for not having a local hukou ^[15][39]. Chan highlights how some local governments have used the hukou system to exploit and discriminate against migrants, regardless of intention, contributing to social gaps between urban and rural populations ^[9][11]. According to Zhou and Hui, the hukou system favors wealthier migrants and the highly skilled and educated ^[13][38]. Another unintended effect of the hukou was the separation of parents and children over long distances for long periods, similar to transnational families in other parts of the world with similar possible negative effects ^[16][17][40,41]. Some regions have had more support for urbanizing, resulting in economic growth and wealth becoming more concentrated ^[5][9][14][10,11,28]. Where others have theorized the effects of China’s hukou on migration choices, Zhou and Hui provide empirical evidence detailing how it influences self-selection for migrants ^[13][38]. They identify inter-regional selection patterns and describe how immigration policies create “segmented citizenship,” hindering undocumented migrants’ ability to integrate fully. Urbanization and migration have also produced social mobility, moving many families into the middle class, but the urban–rural wage gap remains ^[5][28].

Historically, scholars have considered employment as one of the most prominent factors driving internal migration ^{[18][19][20][21]}[1,6,42,43]. Some people move if there is a better job or better pay elsewhere ^[12][13][37,38]. Many former rural residents work in Chinese cities and special economic zones in better-paying jobs, often in manufacturing ^[11][14][10,36] for U.S.-owned companies and for the U.S. market. Therefore, these internal population trends are related.

In examining the continuous decline in U.S. migration, there are disparities in the rates of decline between certain types of relocation and population segments. Annual moves have diminished most at the local level (same county) and among renters ^[22][23][3,12]. Local moves within a county are more likely to be related to changes such as marriage, growing families, and changing housing needs. These are life course events that have shifted as more recent generations wait longer to pursue them.

For the first time in well over a century, young adults (age 18–34) in the U.S. were most likely to live with a parent as of 2014. Chatterji et al. determined that the Affordable Care Act, with its provisions allowing children to stay on their parents’ insurance plan until age 26, had a substantial effect on the likeliness of young adults living with a parent ^[24][44]. Millennials between the ages of 25 and 34 account for a disproportionately high percentage of moving adults, but the rate at which they make short-distance moves is the lowest in twenty-five years ^[22][3].

Life cycle events affect the labor migration behavior of rural families. The probability of migrant work is higher during the family formation period (the period from marriage to first childbirth). During the expansion of the family, the pressure of child-rearing reduces the probability of migrant work. As the family enters the stable period and the children enter the labor force, the probability of migration increases again ^[25][26][45,46].

According to data from the Chinese Ministry of Civil Affairs, marriage rates in China have declined drastically since 2013. After increasing from 6.3% in 2005 to 9.9% in 2013, the marriage rate dropped to just 5.8% in 2020, the lowest in at least 20 years. Some argue that internal migration has made it harder for people to marry, as the income of migrants is lower compared to long-term urban residents, and cultural differences appear between them and the unmarried people in the villages.

The aging population also plays a role in China’s internal migration patterns. There is a significant inverted U-shaped relationship between age and migration ^[27][28][47,48]. Zhang and colleagues constructed a life table of rural migrant workers and found that from the age of 16 to 41, migrant workers tend to continue working in cities. After the age of 41, the number of migrant workers remaining in cities sharply decreases as they begin returning to family in their locations of origin ^[29][49]. Aging, along with a fertility rate under replacement rates, will likely slow internal migration in the years ahead.

Cities play a prominent role in migration patterns. “Fast locations”—those with high resident turnover—produced much of the United States’ migration in the late 20th century as they experienced rapid growth ^[30][2]. These included metros such as Las Vegas, Phoenix, and Miami. As the number of native-born people increased in these locations and populations’ roots grew deeper, with more people identifying as “from there,” attachment increased, and gross out-migration declined. Coate et al. argue that growth in “home attachment” accounts for a majority (roughly two-thirds) of the United States’ internal migration decline ^[30][2].

Furthering the notion of home attachment, research shows that most adults live near their mothers for most of their lives ^[31][50]. There are also racial differences in the distance from their mothers at which Blacks and Whites live, with Black adults living significantly closer (<5 miles) than White adults (approximately 10–15 miles), and demographic and socioeconomic factors accounting for much of the difference ^[31][50].

Schewel ^[32][17] argues that there is a “mobility bias” in most migration studies. Scholars consistently focus on migration patterns and their catalysts, overlooking factors that create immobility and identifying themes for those who do not move or are stuck in place. Both economic and noneconomic determinants influence people’s propensity to move and contribute to racial and ethnic differences in migration. Minority households are less likely to migrate internally across relatively long distances that cross state or county borders. However, Black and Hispanic households are much more likely to migrate within the county than White households ^[33][51]. Internal migration also increases with education, leaving workers who are less “skilled” and less educated stuck in cities with high unemployment rates ^[34][35][52,53].

Huang and Butts proposed the concept of segmented immobility, arguing that internal migration is more relational than geographic because most people want to live near those who are similar to them along lines that are important to them ^[36][4]. They analyzed the impact of political partisanship, urbanization, and race on intercounty migrations from 2011 to 2015 and found that these factors immobilized migration between dissimilar counties, a significant finding as the U.S. becomes more politically divided, with a large percentage of the U.S. population living in neighborhoods comprised almost entirely of residents who vote like them ^[37][54].

In China, the mobility of some citizens is restricted based on their inability to become homeowners or live in certain areas because of their hukou status. In some cities, the requirements to obtain local hukou are unattainable ^[7][13][33,38] and can deter potential migrants. In the U.S., rising costs of living contribute to where people migrate ^[38][55] and may factor into whether people choose to relocate or even have the ability to do so. A study conducted by the Federal Home Loan Mortgage Corporation indicates there have been steep declines in affordable (unsubsidized) housing for very low-income renters. In costly states such as California, even median-income households saw the number of apartments considered to be affordable cut nearly in half between 2010 and 2016 ^[39][56]. A few years ago, the top 10% of counties with the highest housing costs (by % of income) accounted for one-third of the United States’ population growth. In 2021, those counties saw net losses in population, while counties with more modest housing costs experienced population growth ^[38][55].

Therefore, while international migration is relatively rare, less than 4% of the world's population, internal migration is constant. Rural-to-urban migration causes urbanization, but also internal migration patterns, even between cities and metropolitan regions, can have large social, economic, and political consequences for countries. Furthermore, something that escaped many researchers in the past is that internal migration patterns in a given country may be caused by policy decisions happening in other countries (e.g., offshoring, neoliberalism, colonization, de-industrialization) or how internal migration leading to industrialization abroad can have large effects locally such as increasing inequality, creating more complex supply chains, or creating certain dependency on imports. Thus, studying international and internal migration in tandem with demographic and socio-economic changes can give researchers a better sense of global systems and dynamics.