System Dynamics – intro only

Copyright 2019 Graham Berrisford. One of about 100 papers on the System Theory page at Last updated 22/02/2019 11:51


Ashby’s discrete event-driven system is transformable to one of the kind modelled in Forrester’s System Dynamics.

The trick is to abstract from singular entities to populations of them (stocks), and from singular events to batches of them (flows).

The stocks can be resources of any kind – materials, energy, organisms, happiness, whatever.

The flows represent interactions between stocks (rather than within a stock).


System Dynamics

Stock and flow models

<create and use>                 <idealise>

System modellers <observe & envisage>  Interdependent quantities


Consider two stocks: the quantity of sheep and the quantity of wolves.

A System Dynamics model can help to explain how these stocks interact, using the idea of causal loop.


A growth in the stock of


will increase the stock of


will deplete the stock of



Stocks and flows can be represented in a causal loop diagram (CLD).

Here for example is a CLD relating resources to human population

Note that it omits economic growth (or GDP) and the decrease in birth rate that it causes.


Animating the model reveals the trajectory of changes over time to the size of a group, stock, population or resource quantity.

It reveals how feedback loops and time delays affect the stock levels over the long term.

So helps to answer questions like:

·         "Will the system settle down to a steady state?

·         “What steady states are possible?"

·         “Will the system crash or halt?

·         "Does the long-term behavior of the system depend on its initial condition?"


A System Dynamics model is closed, a self-contained model of stock populations that increase and decrease in response to flows

However, each stock may be seen as a system or subsystem in its own right, and the whole model may be regarded as an ecology.


In discussing System Dynamics, it is important to distinguish theory, animated theory and reality.


Abstract System Dynamics model

A theory: a model of stocks and flows that interact according to fixed laws

Concrete System Dynamics model

An animated theory: a performance of actions precisely according to the laws above

An ecology in the real world

Actors that interact to a greater or lesser extent according to the laws above


When a model of a human social system is animated, the actors robotically obey the roles and rules (modelled as stocks and flows).

In the real world equivalent, human actors may ignore or disobey those roles and rules.

They may spend most of their time acting outside of the modelled system and/or playing roles in other systems, even ones with conflicting rules.

Which to say, there may be a mismatch between theory and reality.


Further reading

Read these short modules.

·         System change

·         System state change and regulation by causal loops


For more, read “System Dynamics” on the system theory page.

And here are links to a couple of Gene Bellinger’s videos.