When the closed succession of causal influences tends to reinforce or amplify the condition that triggered the succession, it is called a positive-feedback loop. Such loops have already been encountered and diagrammed in our analysis of the wage-price spiral associated with inflation (p. 4.11:50) and of the effects of subsidies on "risky" behavior (p. 4.11:159). In our graphs they are represented by a closed chain of arrows and a plus sign.

Virtually any complex, real-life system includes fluctuations, often called "noise," arising from so many different sources that they are usually treated as random. While negative-feedback loops tend to protect the system from these fluctuations (as in our thermostat example), positive-feedback loops sometimes exacerbate them until they interfere with the system's proper function. Imagine, for example, a bus system consisting of a series of buses, which travel along a fixed route, picking up and letting off passengers at a large number of stops:

• Each bus takes about eight hours to complete the route, and consecutive buses are supposed to run a half hour apart.

• If no one is getting off or waiting to board at a given stop, the bus does not stop there.

• A published bus schedule, indicating the time when each bus is supposed to reach each stop, is used by most passengers. Some passengers, however, do not consult the schedule, but simply wait for the next bus to arrive.

• Even passengers who consult the schedule may arrive at their departure stop a little early or late.

• The schedule, which is rather tight, was determined by noting the average times required by a bus to travel from each stop to the next. The drivers are directed to drive at the speed limit, so far as traffic and safety conditions permit.

	•	Each bus takes about eight hours to complete the route, and consecutive buses are supposed to run a half hour apart.
	•	If no one is getting off or waiting to board at a given stop, the bus does not stop there.
	•	A published bus schedule, indicating the time when each bus is supposed to reach each stop, is used by most passengers. Some passengers, however, do not consult the schedule, but simply wait for the next bus to arrive.
	•	Even passengers who consult the schedule may arrive at their departure stop a little early or late.
	•	The schedule, which is rather tight, was determined by noting the average times required by a bus to travel from each stop to the next. The drivers are directed to drive at the speed limit, so far as traffic and safety conditions permit.