Systems Thinking for 21st-Century Cities: A Beginner’s Introduction — Part #3

Stocks, flows, and feedback loops

In Part #1 of our series, we explore why systems thinking for cities matters and what it means. In Part #2 of our series, we look at how systems, subsystems, boundaries, and lenses can be applied to a city’s wicked problems. In this article, Part #3, we’ll take a closer look at stocks, flows, and feedback loops in relation to city systems.

Stocks and Flows

Within a system, stocks are elements of the system that you can see, feel, count or measure. Flows cause stocks to change by accumulating or depleting. As depicted in the image above, stocks and flows are commonly explained in systems thinking through the bathtub model. If the bathtub is a system, the water is a stock, while taps and drains facilitate the flows of water (inflow by tap, outflow by drain) in the system.

Daniel Aronson explains on The Systems Thinker, “a technique commonly used to distinguish a stock from a flow is to consider what would happen in the system if time were to stop. Stocks, which are accumulations, would continue to exist. Flows, however, would disappear because they are actions.”

In the context of cities, we can start to understand a city’s comprehensive system and subsystems when we take a look at the city’s stocks and flows.

A city’s stocks can include any element that you can see, feel, count or measure, whether they are biological, physical, social, and/or cultural. I list a few examples in the image above — people, money, policy, power, stories, ideas, food, energy, love, water, and so on. Stocks accumulate (increase) or deplete (decrease) over time; in other words, they are constantly changing or in flux depending on the flows that affect them. For example, a city’s population will increase or decrease based on births, deaths or relocation. The city’s economic GDP can decrease, safety can increase, water access can decrease — all based on a variety of complex, interdependent flows.

A city’s flows are the mechanisms, actions, or events that cause a city’s stocks to accumulate (increase) or deplete (decrease) over time. Stocks are subject to not just one flow, but many flows. I list a few examples of flows in the graph above — births/deaths, investment, legislation, brainstorms, droughts, migration, harvesting, and so on. Flows are constantly happening in predictable and unpredictable ways. For example, birds migrate every winter, but droughts depend on that year’s climate conditions.

Stocks and flows constantly interact, affect each other, and change. To better understand this relationship, we must also understand feedback loops.

Understanding feedback loops

Feedback loops are a return of information within a system that subsequently affects a system’s flow and stock. The two main types of feedback loops are 1) reinforcing feedback loops which compound change (accelerating more of the same in one direction), and 2) balancing loops which seek equilibrium (constraining rampant change in either direction).

For cities, the population is an important stock of the system. A city’s population has traditionally been viewed as one measurement of a city’s health — with increasing population seen as positive growth, and decreasing population seen as negative decline. We can see how feedback loops — reinforcing or balancing — could affect a city’s population.

Reinforcing feedback loops compound more of the same and be can be positive or negative, creating virtuous cycles (positive) or vicious cycles (negative). In the case of a virtuous population cycle, an increase in population leads to further increase. For example, an increase in population would lead to more births in a city, people attracting more friends/family to move to the city, a positive reputation that would attract relocation to that city, etc. On the other hand, a vicious population cycle could lead to population decreases for the complete opposite reasons.

Balancing feedback loops are goal-seeking feedback loops that aim to prevent extreme change in either direction and to keep a system balanced to achieve its goal. In the case of population, a city might have a goal to increase population (i.e. a stock of people) through interventions (i.e. a change in flow) such as policies to support immigration, economic opportunity to retain university talent, investment in K-12 education to retain young families, etc.

Understanding how city stocks and flows change with feedback loops provide systems thinkers the building blocks to then understand causality (how one state or process contributes to the production of another state or process) and emergence (how properties or behaviors emerge when parts interact as a wider whole). We’ll explore causality and emergence in the next article — Part #4.

Thanks for reading & get in touch

This is article #3 in a multiple-part series, ‘Systems Thinking for 21st-Century Cities: A Beginner’s Introduction.

Part #1: Systems Thinking for 21st-Century Cities: A Beginner’s Introduction — Part 1 (why it matters, what it means, and 3 ways to start)

Part #2: Systems Thinking for 21st-Century Cities: A Beginner’s Introduction — Part 1 (how city systems, boundaries, and lenses relate to wicked problems)

Up next: Systems Thinking for 21st-Century Cities: A Beginner’s Introduction — Part 4 (causality and emergence)

Systems thinking is a vast and complex field that has been pioneered by many leaders and experts. The goal of this series is to explore how common systems thinking concepts can apply to cities and wicked problems in a manner that is accessible and thought-provoking for beginners.

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“No one can define or measure justice, democracy, security, freedom, truth, or love. No one can define or measure any value. But if no one speaks up for them, if systems aren’t designed to produce them, if we don’t speak about them and point toward their presence or absence, they will cease to exist.” — Donella Meadows