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Stock and Flow Reasoning

Why It Matters

Most variables we care about are either stocks or flows, and they behave differently. Policies that work on flows may not work on stocks, and vice versa.

The Key Move

For any variable, ask: is this a stock (accumulated quantity at a point in time) or a flow (rate per unit time)? Stocks can only change through inflows and outflows. Stocks integrate flows; flows are derivatives of stocks.

Classic Application

Bathtub dynamics. Water level (stock) changes only through faucet (inflow) and drain (outflow). You can't directly set the level - you can only adjust flows and wait.

Surprising Application

Reputation. Trust is a stock. Actions are flows that add or subtract. You can't directly set your reputation - you can only take actions that accumulate over time. Stocks have momentum; they change slowly even when flows reverse.

Failure Modes

  • Expecting stocks to respond immediately to flow changes
  • Treating stock variables as directly controllable
  • Ignoring that stocks often have outflows you don't control (depreciation, forgetting, decay)

Go Deeper

Meadows' "Thinking in Systems." System dynamics literature.