SYSTEM DYNAMICS

Dr. John Bradford

‘System-as-Cause’ Thinking

1. Laundry-list approach– Gravity, – Removal of Hand

2. System-as-cause approach:

– …. ?

What causes the Slinky to oscillate?

‘System-as-Cause’ Thinking• What causes the Slinky to oscillate?• THE SLINKY!

– The system itself is always the cause of its own behavior. – “Mental models should contain only those elements

whose interaction is capable of self-generating the phenomenon of interest" (Richmond 2010: 6).

Feedback

• Feedback Loop: occurs when changes generate effects that then influence the original causes of the change, making subsequent change either more (+) or less (-) likely.

CauseEffect

Feedback

Two types of Feedback: 1. Positive (reinforcing, amplifying):

Initial changes become amplified or magnified over time; patterns are reinforced.

– Examples: exponential population growth; nuclear explosion; ‘rich getting richer’, etc.

2. Negative (counteracting, balancing):

Initial changes are counteracted or balanced out, so that conditions remain relatively stable.

– Examples: homeostasis; a thermostat; “what goes up, must come down”, etc.

Births+

Population+

Force of Gravity

-

Jump up+

Positive Feedback

Negative Feedback

Causality and Feedback

• Feedback and Self-reinforcing patterns– Social phenomena can appear to take on a life of

their own. How does this happen?– Initial changes generate (other) conditions

which encourage the original effect.

Original Effect

Alternative Causes

Original Cause

Causality and Feedback

• Feedback and Self-reinforcing patterns– Example: English Colonialism spreads English

throughout the world. England ceases to be a colonial power, but English is still a popular world language because English is spoken in the United States, which became a world power.

Global English

US speaks English

EnglishColonialism

Causality and Feedback

• When we think of causal relations, we normally think of these 3 possibilities:1. A may cause B2. B may cause A3. A and B may both be caused by some other

factor C

Causality and Feedback

• There are also 3 additional explanations, which we usually forget:4. A and B may cause each other (with no initial

prompting other than random fluctuations)5. A and B may cause each other, once initially

activated by some other factor C6. A and B may cause each other, in a process of

mutual feedback with some other factor (or set of factors) C

Causality and Feedback

Irreversible Processes• Social research usually assumes that processes are

reversible: “if a change in X makes Y change in a certain direction, then the opposite change in X would generate an opposite change in Y.”

• Example: Poverty and Crime– Increases in poverty (X) tend to be associated with increases in

crime (Y). – Assumption: “Therefore, a decrease in relative poverty will

generate a decrease in crime.”– Even if the original cause was poverty, reducing poverty may or

may not reduce crime!

Causality and Feedback

Irreversible Processes• Analogy:

– if someone is shot by a gun, removing the bullet won’t make the wound go away.

‘System-as-Cause’ Thinking

Four assumptions that are usually wrong: 1. *Causes operate independently of each other: (“laundry-list” thinking)2. Causality runs one-way: (no feedback)3. Effects are “linear” (fixed or proportional to

their effect)4. Effects are instantaneous (no lags or delays)

Stocks and FlowsSt oc k

f lowing

Stocks “Nouns” that indicate conditions or states of

being at a point in time. Stocks are things that accumulate over time from flows

Flows “Verbs” that represent activities or processes,

which exist over time. Flows fill and drain stocks, that is, they update the

magnitude of stocks.

Stock-Flow diagram of Feedback Stock-flow structure of “Positive” (Reinforcing) and

“negative” (counteracting) feedback systems:

Reinforcing Loop: Counteracting Loop: Exponential growth Exponential decay

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A feedback loop occurs whenever a change in the magnitude of a stock in turn affects a flow into or out of that same stock.

Feedback loop Dynamics When both positive and negative feedback are present in

the same system, four possibilities arise:1. exponential growth: the reinforcing loop will dominate the

counteracting loop.2. exponential decay: the counteracting loop will dominate the

dominate the reinforcing loop.3. equilibrium: they balance each other out.4. Oscillation: it goes up, then down, up and down, …

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