System behavior describes how systems work and the ways in which systems can change over time.

## What Is System Behavior?

Next, we’ll understand how systems behave over time, by considering stocks and flows. This forms the basic foundation that lets us build up into more complex systems.

### Stocks and Flows

A stock represents the elements in a system that you can see, count or measure. It can be commonly thought of as an inventory, a store, or a backlog.

Flows are the means by which the stocks change over time. Inflows increase the level of stock, while outflows decrease the level of stock.

#### 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 with system dynamics:

• 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 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.”
System Behavior: How Systems Change Over Time

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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 .

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• 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.