Monday, February 22, 2016

Cities as social reactors

Fig 1. Inka storehouses

Some traits of cities seem universal, shared by all cities, large and small, past and present. Neighborhood organization is one of these urban universals, something I have talked about in previous posts (here, and here, for example).  See Smith et al. (2015). Other traits seem more limited to only some kinds of cities, or cities in only a few regions or periods. For example, Inka cities were full of standardized stone storage facilities, built and maintained by the state (Figure 1); such storehouses are rare in other cities. Contemporary cities have organized trash pick-up (whether government or privately run), while past cities lacked this institution. One of my reasons for writing this blog is to explore the nature of these similarities and differences among cities across the span of deep history and around the globe.

A number of lines of research now suggest that that the role of cities as “social reactors” may be another urban universal. This means that the presence of greater numbers of people—in larger settlements—leads to more social interactions, which in turn produces social and economic benefits far beyond what smaller settlements can produce. Cities are places where individuals interact socially in all kinds of ways. As the number of people in a city goes up, the number of potential interactions with other people also rises, but it rises much faster than the number of people. The increase is exponential (Figure 2). These interactions have been called "buzz" (Storper and Venables 2004).
Fig 2. Interactions & pop.

A long line of research in the social sciences focuses on the negative consequences of growing city size. More people means more crowding, and crowding in cities can cause stress, both psychological and social. Larger cities have more crime and more poverty. Social relations may be less personal in larger cities, with neighbors less willing to help one another. One intriguing theory, by archaeologist Roland Fletcher (1995), posits thresholds of population size and density that growing settlements cannot cross unless they develop new means of communication and social integration (Figure 3). Otherwise the stresses of living with too many people make life intolerable and settlements will disintegrate or break down.  When early village dwellers figured out how to communicate information with writing, and how to divide their settlements into increasingly specialized spaces and buildings, they could achieve the transition to the first cities, which had more people and denser populations.
Fig. 3. Fletcher's book
Growing city size also has its positive consequences. In early times, the most basic attraction of city life was protection. War and violence have been prominent in all eras of history. Once our ancestors adopted agriculture and settled down, the concept of property was born. Property and riches led to crime and warfare. People quickly figured out that there was strength in numbers. Kings and rulers in past times wanted their subjects to move into town. People were easier to tax and control in cities, and their economic activity created growth and prosperity that benefitted the rulers.

Now let's jump to the present. In contemporary cities, urban growth has what economists call “agglomeration effects” (Figure 4). This means that as firms and factories set up in cities, they attract workers and create greater specialization and increased productivity. These in turn generate growth, attracting even more people to cities and creating more output. The spatial concentration of people and firms in cities leads to economies of scale, which pushes urban productivity even higher. Knowledge and skills can be transferred among people, and from one industry to another, because of their concentration in a city.
Figure 4.  Agglomeration effects
This picture of agglomeration economies is a pretty standard view of contemporary cities, as seen by urban economists and economic urban geographers. It is easy to see how cities in this sense can be called “social reactors.” They are places where concentrations of people, firms, and institutions create wealth and growth, built on a foundation of the way the people interact with one another. This is definitely the way cities operate within modern capitalist economies. But what about cities before the modern era, before the birth of capitalism? They didn’t have wage labor; workers did not move among firms and industries as they do today; land markets were rudimentary or non-existent; and economies were far less dynamic than they are today.

Were cities in the past also “social reactors”? My answer to this question a few years ago would have been “no.” It seemed to me (and to many urban scholars) that the generative properties of cities—their capacity to grow quickly and expand economically, their ability to create wealth—arose from the capitalist economy. Ancient cities did not have agglomeration economies, and thus their roles as social reactors must have been far less important than today.

But then I started working on urban scaling with three colleagues. The results have been surprising and illuminating to me, and important for a general understanding of urbanism. I’ve blogged a bit about the scaling work previously (an early post, and later), and I will talk about it more in the future. For now, I will say that this research had convinced me that the concept of social reactors definitely applies to ancient cities, and it even applies to large villages and towns. Something fundamental happens when people live together in settlements, and these fundamental processes become stronger as a settlement gets bigger.

Here is a very brief description of three reasons why I changed my mind on ancient cities and settlements as social reactors.

First, the very regular patterns of how urban traits change with city population, as found for contemporary cities, also apply to past cities. These relationships are analyzed with scaling models, and our research group has found that the scaling patterns of modern cities are also present in cities before the modern era.
Fig. 5. Mandan village
Second, these scaling relationships are also found in village settlements among tribal peoples (Figure 5). To me, this is an astounding finding. Generative processes that are thought to be “urban” in nature also affect much smaller settlements in societies that lack cities, kings, laws, and social classes. Wow, this is nothing short of incredible.

Third, the explanation for the above reasons comes from Luis Bettencourt’s (2013) formal model of scaling. Rather than tracing the city dynamics that create scaling to agglomeration economies, this model traces them to face-to-face social interaction within a structured built environment. This means that they should be present in historical cities, in ancient cities, and in non-urban village settlements. And they are!

After a few years of working groups and meetings our project now has a name: “The Social Reactors Project: Human Settlement and Networks in History” and a website. We are physicist Luis Bettencourt, economist Jose Lobo, archaeologist Scott Ortman, and me. Our website has just gone live this week! Check it out for our work and papers, and for the names of postdocs and students who are helping.

The idea that face-to-face social interactions (buzz) are crucial to cities and to urban life is nothing new. Here are three quotes from diverse authors:

“The central theme of this book is that cities magnify humanity’s strengths. Our social species’ greatest talent is the ability to learn from each other, and we learn more deeply and thoroughly when we’re face-to-face.”
--Urban economist Edward Glaeser (2011:250).

“Again and again, we find that one key to creating social capital is to build in redundancy of contact. … Common spaces for commonplace encounters are prerequisites for common conversations and common debate.”
-- Political scientist Robert Putnam and Civic leader Lewis Feldstein (Putnam and Feldstein 2003:291).

“Stable physical settings” promote community. “The neighborhood accommodates and facilitates social interaction.”
--Urban planner Sidney Brower (Brower 2011:19)

I believe this notion was also crucial to past communities, a theme I develop in my new book, At Home with the Aztecs: An Archaeologist Uncovers their Daily Life.

The innovation of our scaling research group is to demonstrate, with solid quantitative data, that these face-to-face interactions were not only present and important in the past, but that they also generated change and output in the past, just as they do today. We have a bunch of papers in production and in press, and some published (see the website). The role of cities as social reactors is not just a contemporary phenomenon, an attribute of capitalist economies. Rather, it is a fundamental attribute of human settlements through deep history and across space.


Bettencourt, Luís M. A.
2013  The Origins of Scaling in Cities. Science 340: 1438-1441.

Brower, Sidney N.
2011  Neighbors and Neighborhoods: Elements of Successful Community Design. APA Planners Press, Chicago.

Fletcher, Roland
1995  The Limits of Settlement Growth: A Theoretical Outline. Cambridge University Press, New York.

Glaeser, Edward L.
2011  The Triumph of Cities: How our Greatest Invention Makes us Richer, Smarter, Greener, Healthier, and Happier. Penguin, New York.

Putnam, Robert D. and Lewis M. Feldstein
2003  Better Together: Restoring the American Community. Simon and Schuster, New York.

Storper, Michael and Anthony J. Venables
2004    Buzz: Face-to-Face Contact and the Urban Economy. Journal of Economic Geography 4 (4): 351-370.