Chapter 1: The Basics
tis systems-thinking stocks flows feedback-loops
Status: Notes complete
Overview
Every system has three essential components: elements, interconnections, and purpose. Stocks accumulate; flows change stocks; feedback loops close the causal chain back to the stock. This structure — not external events — generates all system behavior.
Three Components of Every System
“A system is an interconnected set of elements that is coherently organized in a way that achieves something.”
| Component | Description | Importance |
|---|---|---|
| Elements | Visible, tangible parts (players, buildings, intangibles like pride) | Least important — replaceable without changing the system |
| Interconnections | Relationships holding elements together; often information flows | Critically important — changing them changes behavior |
| Function / Purpose | What the system actually does; revealed only by observing behavior | Most important and least obvious |
Test for “system-ness”: Can you identify parts? Do parts affect each other? Do parts together produce an effect different from each alone? Does the effect persist across varying circumstances?
“The least obvious part of the system, its function or purpose, is often the most crucial determinant of the system’s behavior.”
Stated goals ≠ system purpose. Actual behavior reveals real purpose. If a government proclaims environmental protection but allocates little money to it, environmental protection is not its purpose.
Stocks and Flows
Stocks
- A store, quantity, or accumulation of material or information built up over time
- Examples: water in a bathtub, population, books in a bookstore, money in a bank, self-confidence
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“A stock is the memory of the history of changing flows within the system.”
Flows
- Flows change stocks over time: births/deaths, purchases/sales, deposits/withdrawals
- Inflows increase stocks; outflows decrease stocks
The Bathtub Principle
- Sum of inflows > sum of outflows → stock rises
- Sum of outflows > sum of inflows → stock falls
- Inflows = outflows → stock in dynamic equilibrium
Critical Stock Properties
Stocks change slowly — they act as delays, buffers, ballast, sources of momentum:
- Even abrupt changes to flows take time to change stocks
- This is why populations can’t stop growing overnight, why pollution accumulates for decades before damage appears
You can fill a bathtub two ways:
- Increase inflow rate (discover more oil)
- Decrease outflow rate (burn less oil — equally powerful but often overlooked)
Stocks decouple inflows and outflows:
- Gasoline in a tank lets drivers fill up at different times than they drive
- Banks let you earn money at a different rate than you spend it
- This decoupling is why inventories, reservoirs, and savings exist
Feedback Loops
“A feedback loop is a closed chain of causal connections from a stock, through a set of decisions or rules or physical laws or actions that are dependent on the level of the stock, and back again through a flow to change the stock.”
Balancing Feedback Loops (B)
- Goal-seeking / stabilizing loops
- Oppose whatever change is imposed on the system
- Examples: thermostat, coffee cooling, body temperature regulation, bank account management
- If stock pushed too far up, loop pulls it back; if pushed down, loop pushes it up
“Balancing feedback loops are equilibrating or goal-seeking structures in systems and are both sources of stability and sources of resistance to change.”
Reinforcing Feedback Loops (R)
- Self-enhancing, amplifying loops
- Generate exponential growth or runaway collapse
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“The more is there, the more is added.”
- Examples: compound interest, rabbit population, soil erosion cycle, arms races
- Rule of 70: 70 ÷ growth rate (%) = years to double
Multiple Loops
- Real systems have multiple interlocking loops pulling stocks in different directions
- Which loop dominates determines current behavior
- Shifting dominance: as conditions change, different loops take control → explains complex behaviors
Key Insight
“The concept of feedback opens up the idea that a system can cause its own behavior.”
Instead of: A causes B — ask: Does B also cause A? How does B reinforce or reverse itself?
Move from seeing a static world to a dynamic one. Stop looking for who to blame; start asking “What’s the system?”
Last Updated: 2026-05-30