Hard to define

• Lots of definitions that vary across locations and disciplines
• Usually some combination of population, spatial extent, and construction
• Differences in definitions make huge differences in interpretation

Global urbanization: Old United Nations Population Division approach

Based on countries’ individual definitions of rural and urban.

Global urbanization: European Commission approach

Defines cities as clusters of 1-KM^2 areas with at least 300 people, adding up to at least 5,000 people.

However we define them, something amazing is happening

Long-term urban population by region.

And is likely to continue to happen

United Nations Population Division projections to 2050.

So why even ask why we have cities?

• Few species live in such large groups
• High mortality rates in cities until the last century
• Vast majority of global population was rural until this century

How do we understand the question?

• Why do cities exist?
• Why are cities where they are?
• Why do cities persist?

The evolutionary story

Humans were “preadapted” to make the most of cities

• Tool use and construction
• Large social networks (~150 people)
• Effective 3-D navigation

Food, water, and material supply

Military defensibility

Xi’an city wall, Aaron Zhu, Wikimedia Commons

Trade linkages

Map of the Silk Road, Wikimedia Commons

Political coercion

Persepolis, in modern-day Iran, by Delso, Wikimedia Commons, 2016

Agglomeration economies

Benefits of concentration increase as more people gather

• Specialization: If similar economic activities cluster together, the inputs to those activities can also cluster, reducing costs
• Transaction costs: Easier to match labor to jobs, lowering costs of labor and job search
• Knowledge sharing: Easier diffusion of new ideas and techniques
• Economies of scale: Lower cost in public goods provision per person if population is concentrated

Settlement scaling theory

• Argues that human productivity increases with human interactions”¹
• Evidence suggests that productivity increases faster than population, while infrastructural needs increase more slowly than population²
• Increasing benefits and decreasing costs incentivize urban living

Example: Settlements in Roman Britain

Map of settlement sites (Ortman, et al., Science Advances, 2024)

Example: Settlements in Roman Britain

Relationship between settlement size and coin use

Example: Settlements in Roman Britain

Relationship between settlement size and fine good use

Example: Northern Rio Grande Pueblos

Pueblo sites map (Ortman & Lobo, Science Advances, 2020)

Example: Northern Rio Grande Pueblos

Measures relative to population. Diversity refers to diversity of tasks per captia, so it’s sort of the inverse of specialization.

Review

• No clear consensus on definition of a city
• Under any definition, recent urbanization is a major historical shift
• Certain evolutionary features of humans make cities possible
• Geographic factors like resources, trade and political power shape cities’ location
• Settlement scaling theory suggests cities benefit from increased economic productivity and decreased infrastructure costs