1. The system itself, not individual components, causes the behavior of the system. The interaction of elements in a system can self-generate phenomena through feedback loops. 2. There are two types of feedback loops: positive/reinforcing feedback which amplifies changes, and negative/balancing feedback which counteracts changes to maintain stability. 3. When both positive and negative feedback are present, it can lead to exponential growth, exponential decay, equilibrium, or oscillation depending on which type of feedback dominates.

1. SYSTEM DYNAMICS
Dr. John Bradford

2. ‘System-as-Cause’ Thinking
What causes the Slinky
1. Laundry-list approach
to oscillate?
– Gravity,
– Removal of Hand
2. System-as-cause
approach:
– …. ?

3. ‘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).

4. 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.
Effect Cause

5. Feedback
Two types of Feedback: Positive Feedback
1. Positive (reinforcing, amplifying):
Initial changes become amplified or
magnified over time; patterns are Population Births
reinforced. + +
– Examples: exponential population
growth; nuclear explosion; ‘rich getting
richer’, etc.
2. Negative (counteracting, Negative Feedback
balancing):
Initial changes are counteracted or
Force of
balanced out, so that conditions remain Jump up
Gravity
relatively stable. +
-
– Examples: homeostasis; a thermostat;
“what goes up, must come down”, etc.

6. 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.

7. 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.

8. Causality and Feedback
• When we think of causal relations, we
normally think of these 3 possibilities:
1. A may cause B
2. B may cause A
3. A and B may both be caused by some other
factor C

9. 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 C
6. A and B may cause each other, in a process of
mutual feedback with some other factor (or set
of factors) C

10. 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!

11. Causality and Feedback
Irreversible Processes
• Analogy:
–if someone is shot by a gun, removing
the bullet won’t make the wound go
away.

12. ‘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)

13. Stocks and Flows
Stock
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.

14. Stock-Flow diagram of Feedback
 Stock-flow structure of “Positive” (Reinforcing) and
“negative” (counteracting) feedback systems:
Reinforcing Loop: Counteracting Loop:
Exponential growth Exponential decay
Populat ion
Populat ion
declining
growing
growt h
decline
rat e ~ rat e
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.

15. 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, …
Population
growing declining
growth
rate decline
~ rate