Why is so much urban growth happening? Demographers sometimes identify push factors driving people from rural areas, such as the collapse of rural economies or political instability. Also important are pull factors, as the positive benefits of being in a city draw people to migrate to the city [11]. Regardless of whether push or pull factors dominate, a growing fraction of people live in cities because they are a useful way of structuring society, what one writer has called “our greatest invention” [4]. The core ingredient of cities’ success seems to be how cities enable greater proximity of people and firms [12]. Proximity promotes interaction and speeds the pace of life, leading to individual and social benefits [13,14,15].

Aristotle famously referred to humans as a social animal [16], by which he meant that our unique skill and love for interacting with one another is part of our species essence. In cities, one could argue we are creating the perfect space for social interaction. If man is a social animal, then cities with their density and proximity are our ideal habitat. Cities could therefore be seen as quintessentially human, an expression of our deep need for social interaction.

Urban benefits to production

There is a voluminous literature on the benefits of proximity to individuals and firms, which is sometimes called agglomeration economics [12, 17]. A large fraction of the urban economics literature focuses on the benefits of agglomeration to production. There are many ways to classify these benefits, but one useful classification focuses on the benefits of Sharing, Matching, and Learning [18, 19].

Proximity enables sharing of infrastructure and resources. Transportation infrastructure, such as roadways or train lines, makes more economic sense when multiple people can use the infrastructure. For trade, facilities like seaport or airports are essential, and their high cost of construction is only economical when multiple entities can help pay for and use them [20]. Sharing benefits can also occur within a firm, particularly one that exhibits increasing return to scale in production. A firm that finds it can produce goods more efficiently by sharing expensive machinery among different production processes may find that concentration of production in one or a few locations is preferable to dispersed operations [19].

Second, the proximity of people and firms facilitate matching [19, 21, 22]. Having multiple firms in the same sector located in one area leads to a pool of workers qualified to work in that sector. This thick labor market makes it easier for firms to find qualified people for specialized jobs. Matching can also occur between firms. For instance, when one firm’s output good is another firm’s input good, proximity can aid in matching up firms, reducing the transaction costs of commerce.

Third, the proximity of people and increased potential for face-to-face interaction promotes learning [23,24,25]. Despite the rise of technologies that allow communication at a distance, such as the telephone and the Internet, research shows that some kinds of “sticky” knowledge is best transmitted through direct interactions [26]. Much of the research into this topic has focused on the development and history of certain innovation clusters, such as the rise of the computing industry in Silicon Valley, where personal interactions among high-tech workings has been shown to be key to knowledge generation [27]. Empirical evidence that look at rates of patent generation and other metrics of innovation show that larger cities generally have higher rates of innovation [13], consistent with the idea that increased interaction leads to more learning.

Urban benefits to consumption

Historically, the focus of urban economics was on the benefits of agglomeration to production. However, in recent decades an increased research focus has been on cities as centers of consumption [28, 29]. Living in a city brings with it access to many amenities and services that are not as frequently available in rural areas: restaurants serving all variety of food, good museums and theaters, etc. Indeed, parks and street trees can be viewed as a type of amenity, part of the overall attractiveness of cities [30], although later in this paper we argue that the evidence suggests such natural features have a greater role to play than as a mere amenity.

Some economists have taken these arguments further, arguing that consumption benefits are now a key reason for urbanization. A famous paper by Glaeser and colleagues called Consumption City [28] noted that rents in cities have grown faster than wages, suggesting workers interest in living in cities has grown faster than firms’. Reverse commuting has grown as a phenomenon, with people choosing to live in the city center for the cultural life and consumption possibilities it offers, even if it is farther from their job and has higher rent than a more suburban location. While urban life has its stresses and difficulties, it is worth remembering that a sizable fraction of people prefers living in cities over the alternative lifestyle in rural areas.

An urban world, ready or not

These various benefits of cities occur because of proximity and the increased interaction it entails. These economic benefits from urbanization are often unequally distributed within cities, and it is important to note that not all sectors of society necessarily benefit from urbanization. However, the historical record suggests that overall these economic benefits of urban form are strong enough that they drive most societies toward urbanization. Over the last century, almost every country has urbanized as it has economically developed (Fig. 2). Research suggests that the causality here goes both ways: urbanization brings economic benefits and hence higher growth in GDP, while economic growth also appears to encourage urbanization [20].

Fig. 2 Urbanization and economic development. The relationship between the percent urban of a country’s population and its GDP per capita. Urbanization data taken from the UNPD for 2015. Per-capita GDP taken from the World Bank. Some small island states and nations are excluded from this graph, as are nations with missing data Full size image

Some countries have tried to slow or limit urbanization, out of fear of the pace or magnitude of changes it might bring. These policies are often seen as failures, in the sense that urbanization occurred anyway (cf., [31]). The demographic and economic forecasts are clear [1]. Urbanization is coming, and we argue that it is smarter for policymakers to plan for the coming urban century than to fight to stop its arrival. Indeed, the urban century will bring with it tremendous economic benefits.

The urban health penalty

We argued above that cities are quintessentially human, that their form reflects a deep-seated desire for social interaction. But cities are also shockingly inhumane. The concentration of people leads to a concentration of environmental problems, as (among other things) local resource use and waste generation degrade local environmental conditions [32]. Cities create a local environment with far different environmental conditions than the ones we evolved as a species to handle [30]. Thus, in this sense, the urban environment is inhumane, by not being in accord with our organism’s design and capacities.

The urban health penalty historically

Much of urban history can be seen as a struggle to make cities more humane. This is particularly true for health, a focus of many of the largest urban innovations over the last several centuries [33]. Historically, urban dwellers had shorter lifespans than rural dwellers, a phenomenon called the urban health penalty. The term “urban penalty” originated in the study of nineteenth century English demography [34], where urban mortality rates, particularly from communicable diseases like tuberculosis, was substantially higher than in rural areas. Infant mortality rates were also higher in urban areas up until the late nineteenth century. Studies in the United States found a similar pattern, focusing in on the environmental determinants of the “urban health penalty” [35].

One useful framework for thinking about how cities have dealt with environmental challenges to health is the Urban Environmental Transition (UET). Formulated by McGranahan and colleagues [33, 36], the UET argues that often cities have dealt with environmental challenges in a predictable temporal order (Fig. 3). First, cities focused on acute, local (neighborhood-scale) challenges, especially providing clean drinking water and sanitation to remove human waste. Then, cities moved to less acute, more city-level issues, such as air quality. Finally, cities moved to thinking about regional or global problems, such as acid rain or climate change. At each stage, cities often can solve a health challenge while displacing the environmental burden elsewhere outside the city center, following the famous maxim that “the solution to pollution is dilution.” Recent papers have suggested that cities in the developing world are not sequentially moving through these stages, but are tackling multiple transitions at once [37]. Regardless, while the UET may not be a guide to the sequence in which future cities will act, we think the UET is a useful framework for understanding how historically many cities have experienced and dealt with environmental issues.

Fig. 3 The urban environmental transition. As cities get wealthier over time, the type of environmental burdens they face change systematically [33, 36] Full size image

Dealing with sanitation and water quality is one of the first steps in the UET. For instance, London and other cities in England began to tackle this problem in the nineteenth century. These sanitation improvements, coupled with improvements in the nutrition and diet of the urban poor, led to an improvement in human health. Since then, water supply systems and sanitation systems have dramatically decreased mortality rates, particularly infant mortality. This transition to the “Sanitary City” [38] was the first and arguably most important step taken by cities in removing the urban health penalty. Obviously, however, much work remains to be done, as an estimated 15% of urban dwellers globally lack safely managed drinking water sources and 16% of urban dwellers lack access to at least basic sanitation [39]. While these figures are substantially better than for rural areas, that still means that hundreds of millions of urban dwellers lack safely managed drinking water sources and basic sanitation, especially concentrated in informal urban settlements in developing countries.

In developed countries, efforts to address problems from ambient air pollution began later. In London, for instance, the infamous Great London Smog was still severe enough in 1952 to kill close to 12,000 people in 1 week [40]. In the decades since, however, air pollution emission controls have dramatically cleaned up many urban airsheds. London’s concentrations of suspended particulate matter are now less than a fifth of what they were in the 1950s [41]. In the United States, for instance, particulate emissions (PM 10 ) fell by 57% between 1980 and 2016 [42]. As with the situation with water, much work remains to be done of course. Ambient particulate matter pollution still kills more than 3 million people per year, in both rural and urban areas [43].

The urban health penalty today: Obesity and mental health

The urban health penalty has now in aggregate disappeared, as mortality rates are now lower for urban dwellers than rural dwellers [44]. This is particularly noticeable for the infant mortality rate: in developing countries the Demographic and Health Surveys (DHS) show an average of 86 deaths (per 1000 live births) for rural areas, 75 for the urban poor, and 56 for the urban non-poor [45]. That is, even the more than 800 million people who live in informal or “slum” settlements [46] are generally healthier than their rural counterparts. However, two major facets of the urban health penalty remain.

First, one aspect of physical health continues to be worse in cities: obesity and its associated diseases [47]. Worldwide, obesity has more than doubled since 1980, now affecting more than 600 million people [48]. Obesity is most prevalent when there is increased intake of calorie-rich foods and less physical activity [49]. Both conditions are more common in cities, as greater incomes allow more calorie intake and a larger proportion of jobs are sedentary than in rural settings. It is unclear whether it is cities per se however that are responsible for obesity. In the United States, for instance, where automobiles are available to most households, the global average trend is reversed, and rural dwellers are generally more obese than urban dwellers. Indeed, there is evidence that dense urban living in the U.S. is associated with more physical exercise and lower body mass index (BMI) [50].

Second, some aspects of mental health appear to be persistently worse in urban areas than in rural ones, leading to an urban psychological penalty [51]. Urban life has been associated with higher levels of stress than rural areas, as well as with changes in brain function [52]. Sundquist et al. [53] studied more than 4 million adults in Sweden, finding a significant increase in the incidence of psychosis and depression among populations living at higher densities in cities than those living in more rural areas. Similar large-population studies have been done in Denmark [54, 55] and the United Kingdom [56, 57], with each study defining urbanicity slightly differently and measuring different components of mental health. The effect appears to occur in developing countries as well. A Chinese study of rural to urban migrants found a positive association with schizophrenia [58], and a study in Sao Paulo, Brazil, found higher self-reported problems of mental illness in higher density neighborhoods [59]. Interestingly, the effect of urban life on mental health seems to vary depending on age. In children, some studies have reported that urban life appears to be associated with greater frequency of Attention Deficit Hyperactivity Disorder (ADHD) [60], although other studies have found no association [61]. In adults, urban life appears to be associated with schizophrenia [62, 63], as well as greater incidence of psychosis [53].

These studies suggest that the dose-response relationship between urbanicity and the urban psychological penalty is complex, varying by disease and by the urban cultural context [55]. However, there is a clear empirical trend for the higher population densities found in cities to be associated with a greater incidence of mental health problems. In Fig. 4, we assemble information on the fraction of humanity living at different population densities, using the approximate classes used in the Sundquist et al. [53] paper. In 2000, 2.5 billion people lived at densities greater than 800 people per square kilometers (41%), a figure projected to grow to 3.7 billion people (46%) by 2020. Note that these are the same population density categories for which the Sundquist et al. [53] paper showed an increased incidence of depression and psychosis. If this is applied globally, it would imply that close to half of humanity is living at urban densities that significantly increase the risk of mental health problems.

Fig. 4 Population density and the urban psychological penalty. The bar chart shows the total global population living in different population density classes, calculated from the Gridded Population of the World for 2000 and 2020. The red lines show the increase in the hazard rate over baseline for two common mental illnesses, taken from Sundquist et al. [53], who studied 4.4 million people aged 25–64 in Sweden. Shown is the average increase in hazard rates across men and women. Note that Sweden does not have many neighborhoods with population densities above 10,000 people/km2, so the Sundquist et al. results are not directly applicable to the top population density category Full size image

Causes of the urban psychological penalty

The urban psychological penalty has been explained through three categories of causes [cf., 35]. First, cities have a different mix of populations than rural areas. This structural issue makes analyses comparing urban and rural dwellers challenging. Those of lower economic status are sometimes concentrated in certain cities or neighborhoods. So sometimes are marginalized racial or ethnic groups. Since these populations are sometimes more prone to mental health problems, their concentration in cities make urban areas statistically correlated with poor mental health.

Second, in certain cities or neighborhoods urban life (urbanicity) may lead to a breakdown in social cohesion, the willingness of members of a society to cooperate with each other in order to survive and prosper [64]. There is debate in the literature [65, 66] about whether this breakdown in social cohesion is due to urbanicity per se, or whether it simply is statistically more likely to occur in cities. Cities tend to have more single parents, as well as more families with no extended family living nearby. Cities are also commonly a place with more transient or temporary residents, which can also lead to less social cohesion. Since social cohesion is linked to mental health, processes in cities that reduce social cohesion are associated with poorer mental health.

Third, cities have numerous environmental stressors that are associated with poor mental health [67]. The greater ambient noise in urban environments is associated with higher stress levels and mental health issues [68, 69], as is the greater visual stimulation and “light pollution” in cities [70]. It appears that living at higher densities (crowding) is associated with poor mental health, at least when the crowding is with non-family members [71, 72]. However, this crowding is part of what makes cities humanity’s “greatest invention”- higher densities and a faster pace of life are part of what makes cities successful economically [4]. It is ironic that the same phenomenon, cities’ capacity to increase interaction, is what makes cities great and what makes cities mentally inhumane. This “urban psychological penalty” may not be as easily engineered away with grey infrastructure, as were other facets of the urban health penalty (e.g., water-borne diseases).