The Bathtub Analogy: The World’s Simplest System

This article is an excerpt from the Shortform summary of "Thinking in Systems" by Donella H. Meadows. Shortform has the world's best summaries of books you should be reading.

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What is the bathtub analogy? How does it help explain how systems work?

The bathtub analogy is a way to describe a basic system. Basically, you fill up a bathtub, and when it gets too full, you drain it. This is a simple example of a system, but helps you conceptualize larger and more complex systems.

Read more about the bathtub analogy below.

Using the Bathtub Analogy to See Things as Systems

What is a system? A system is 1) a group of things that 2) interact to 3) produce a pattern of behavior.

Many things in the world operate as systems.

  • The human liver is a group of cells that interact to detoxify the blood, among other functions. The liver is, in turn, part of the larger system of the human body.
  • A football team consists of a group of players on the field, each with a specific role that interacts with the others. The team also consists of coaches, support staff, and fans.
  • Within the system of a corporation, people, machines, and information work together to achieve the corporation’s goals. This corporation then takes place within the larger system of the economy.

Systems may look different on the surface, but if they have the same underlying structure, they tend to behave similarly. 

  • For example, consider the bathtub analogy. The bathtub is a simple system. There is a spout that adds water to the system, and a drain that removes water from the system. If the spout adds water faster than it drains, the bathtub will fill and the water level will rise. Conversely, if the drain removes water faster than the spout adds water, the bathtub will empty.
  • Now consider the system of the world population. From a system point of view, it looks similar to the bathtub analogy. The birth rate adds people to the population; death removes people from the population. If the birth rate exceeds the mortality rate, the population will rise. If people die faster than they are born, the population will fall.

In reality, if you look deeper, the world population is a much more complex system—it’s subject to the economy, health science, and politics, each of which is its own complicated system. But from a certain simplified vantage point, the bathtub and the world population behave similarly.

Understanding the underlying system and how it behaves may be the best way to change the system.

Many systems can be explain using the bathtub analogy:

  • In fossil fuels, the stock is the reservoir of fossil fuels. Mining lowers the stock, while natural processes increase the stock.
  • The world population is a stock of people. The population grows with births and shrinks with deaths.
  • Your self-confidence is a stock. It grows with compliments and personal triumphs. It shrinks with project failures and insults.

System Dynamics

Systems like bathtubs and world populations are usually not static. They change over time—the inflows and outflows change, which causes the stocks to change. 

Let’s consider how you can change the flows in a bathtub, by manipulating the inflow and the outflow:

  • The bathtub starts off empty. You plug the drain and turn on the faucet. This causes the water level (or the stock) to rise.
  • When the bathtub is full, you turn off the faucet. The water level stays the same, because water is neither flowing in nor out.
  • You open the drain. The water level starts decreasing.
  • At some point halfway, you turn on the drain again. The water is flowing in at the same rate that it’s leaving, so the water level stays the same.

This behavior can be put on a graph, which visualizes the system over time. 

Systems thinkers use graphs to understand the trend of how a system changes, not just individual events or how the stock looks currently.

The bathtub analogy should be an intuitive model, and it’s simple as it represents just one stock, one inflow, and one outflow. But from this basic example you can find a few general properties of systems:

  • If the inflows exceed the outflows, the stock will rise.
  • If the outflows exceed the inflows, the stock will fall.
  • If the outflows balance the inflows, the stock will stay the same, at a level of “dynamic equilibrium.”
The Bathtub Analogy: The World’s Simplest System

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Like what you just read? Read the rest of the world's best summary of Donella H. Meadows's "Thinking in Systems" at Shortform.

Here's what you'll find in our full Thinking in Systems summary:

  • How the world, from bathtub faucets to fish populations, can be seen as simple systems
  • The key system traps that hold back progress, such as escalating arms races and policy addiction
  • Why seeing the world as systems can give you superpowers in work and life

Carrie Cabral

Carrie has been reading and writing for as long as she can remember, and has always been open to reading anything put in front of her. She wrote her first short story at the age of six, about a lost dog who meets animal friends on his journey home. Surprisingly, it was never picked up by any major publishers, but did spark her passion for books. Carrie worked in book publishing for several years before getting an MFA in Creative Writing. She especially loves literary fiction, historical fiction, and social, cultural, and historical nonfiction that gets into the weeds of daily life.

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