Image courtesy Gabriel Garcia

Cities are much more than a collection of streets, monuments, and parks — they’re patterns of human interaction taking place in space and time.

From traffic on commuter highways to the hum of conversation in cafes, permanent settlements are what SFI researchers have deemed “social reactors:” accelerators of human interactions and their outcomes. And these outcomes can include everything from marketplaces and sewage systems to moral philosophy and cell phones.

That's what members of a working group happening now at SFI are exploring.

“We know in human history there were not towns all the way down,” says External Professor Luis Bettencourt, one member of the working group. “We want to see that transition.”

For thousands of years, humans were mobile hunter-gatherers. But when we started staying in the same place for longer periods, things began to change — and fast. “It appears that everything since then,” according to SFI External Professor and working group organizer Scott Ortman, “is just the playing out of a series of relationships that emerged when people started settling down.”

Permanent settlements lead to increasing population density, burgeoning trade, and trade specialization. If your neighbor grows your food, you can provide him with a violin in exchange. Living in close proximity — and wanting to stay that way — also forces people to develop institutions for conflict resolution and property. Rather than relying on physical distance for security, as hunter-gatherers did, city-dwellers trust that the police, the law, or their contract with their landlord will keep them safe.  All of these habits and institutions energize — or, one might argue, create — healthy economies.

In quantitatively analyzing human settlements from ancient times to the present, Ortman’s group has found something fascinating: the more things change, the more they stay the same. In fact, says Ortman, “in our framework, there’s not much difference between a Neolithic farming village and a modern city.”

The researchers are working with a variety of measures obtainable from the archaeological record, such as the density of potsherds or size of monuments, and also with information from historical records, such as Roman histories and medieval census data, as well as information from ethnographic studies of contemporary hunter-gatherer societies.

The archaeological record presents thousands of “long-term experiments” in which human groups have tried different ways of harnessing these social networking processes — and they do so in ways that differ from our typical experience.

Our concept of commuting, for example, assumes that each city has a downtown surrounded by suburbs. Commuters travel into the city every day and go home at night. But the Pueblo peoples’ traditional social and economic system, for example, involves ceremonial events that are held in different villages on different days — essentially creating many city centers. This “polycentric commuter flow” allows them to gain the benefits of scale despite the smaller size of individual communities.

So, is your commute worth it?

Living outside a city, what you gain in space, you also lose in time: in exchange for your big backyard, you’ll spend part of the day in a car, train or bus. The suburbs don’t even approach a nomadic lifestyle — but sacrificing space for more time bartering, arguing or playing with other humans creates a reactive environment that has, arguably, fueled the modern world.

Many questions remain: Are hunter-gatherer societies “missing” something, or are social reactors something “extra”? How accurately do measurements of potsherds model population growth? The working group will allow researchers to think through the mapping of observable evidence onto their current theoretical framework as a step in investigating how broadly it applies.

This research has applications in urban planning, sustainable development, and the distribution of wealth in ancient and modern societies. However, it may also point inwards, shedding light on humans’ urban impulse.

Given humans’ social nature, are cities inevitable? No, says Bettencourt. “But when they do happen, they create all these changes that make it very difficult to ‘go back.’” The growth and posterity of cities, he added, is a kind of magic. “We want to know how it works! History is an essential source of such evidence.” 

Read more about Agglomeration Economies, Past and Present meeting.

Read more about the social reactors project: