Chapter 2: Quiz — Feedback Loops
This quiz covers the core concepts from Chapter 2. Answer all questions before checking the answer key at the end.
Multiple Choice
1. In a negative feedback loop, the output is fed back to the input in a way that:
a) Increases the signal with each cycle b) Opposes the direction of change, promoting stability c) Always produces oscillation d) Eliminates all variation from the system
2. Which of the following is the best example of a positive (reinforcing) feedback loop?
a) A thermostat maintaining room temperature at 20°C b) Blood clotting, where aggregated platelets release chemicals that attract more platelets c) Your pupil constricting in bright light d) A cruise control system maintaining highway speed
3. The "gain" of a feedback loop refers to:
a) The total output of the system over time b) The ratio of output to input for one trip around the loop c) The speed at which feedback arrives d) The number of components in the loop
4. Oscillation in a feedback system is most commonly caused by:
a) Positive feedback with high gain b) Negative feedback with no delay c) Negative feedback with a significant delay d) The absence of any feedback
5. The concept of "substrate independence" in feedback loops means:
a) Only electronic systems can have feedback loops b) Feedback loops work best in biological systems c) The same feedback structure produces the same dynamics regardless of what the system is made of d) You cannot compare feedback loops across different domains
6. In the 2008 financial crisis, leverage functioned as:
a) A delay in the feedback loop b) A damping mechanism c) A gain multiplier in a reinforcing loop d) A sensor in a balancing loop
7. In the stock-and-flow framework, which of the following is a "stock"?
a) The rate of rainfall b) The amount of water in a reservoir c) The speed of water flowing through a pipe d) The rate at which water evaporates
8. Cognitive behavioral therapy (CBT) for anxiety disorders works, from a feedback perspective, by:
a) Eliminating the feedback loop entirely b) Increasing the delay in the anxiety loop c) Reducing the gain of the reinforcing loop through cognitive reappraisal d) Replacing negative feedback with positive feedback
9. In a sourdough starter, the accumulation of ethanol produced by yeast fermentation eventually slows yeast activity. This is an example of:
a) A reinforcing loop with no limit b) A balancing loop driven by a byproduct of the system's own activity c) A system with no feedback d) A delay without any feedback mechanism
10. Which of the following best describes why a shower with long pipes is difficult to control?
a) The water pressure is too low b) There is no feedback in the system c) The delay between adjusting the tap and feeling the temperature change causes overshooting d) The gain of the system is too low
True or False
11. "Negative feedback" means the feedback has harmful consequences for the system.
12. A system can have both positive and negative feedback loops operating simultaneously.
13. If the gain of a reinforcing loop is greater than one, the system will grow without limit forever.
14. The predator-prey oscillations described by the Lotka-Volterra equations are caused by external shocks to the ecosystem.
15. Deposit guarantees during the 2008 crisis worked by breaking the bank-run reinforcing loop — depositors had no reason to panic if their money was guaranteed.
16. The concept of feedback applies only to engineered systems; biological and social systems use different principles.
17. A key insight of system dynamics is that oscillations in systems like real estate markets can be endogenous — caused by the internal structure of feedback loops rather than external perturbations.
18. In the Richardson arms race model, each nation's military buildup is driven by its own internal goals, independent of the other nation's behavior.
Short Answer
19. Name the four components of a negative feedback loop as described in the chapter. Then identify all four in the example of a central bank managing inflation.
20. Explain in three to four sentences why an arms race produces arsenals far larger than any rational military assessment would justify. Use the concepts of reinforcing feedback and gain in your answer.
21. The chapter describes several systems that oscillate: the shower, the thermostat, the predator-prey cycle, and the boom-bust economic cycle. What is the common structural cause of oscillation in all these systems?
22. Describe one example from the chapter where an intervention successfully reduced the gain of a reinforcing feedback loop. Identify the loop, the gain mechanism, and how the intervention reduced the gain.
23. What is the difference between a "stock" and a "flow" in the system dynamics framework? Why is confusing the two a common source of reasoning errors? Give one example of such a confusion.
24. The chapter claims that a thermostat and a human immune system are "running the same algorithm." In what precise sense is this true? In what sense might it be an oversimplification?
25. Describe the threshold concept of this chapter in your own words. Why does the chapter call substrate independence "a genuinely radical idea"?
Answer Key
1. b) Opposes the direction of change, promoting stability.
2. b) Blood clotting — the aggregation of platelets recruits more platelets, amplifying the response.
3. b) The ratio of output to input for one trip around the loop.
4. c) Negative feedback with a significant delay — the system overcorrects because it is responding to outdated information.
5. c) The same feedback structure produces the same dynamics regardless of what the system is made of.
6. c) A gain multiplier in a reinforcing loop — leverage amplified both gains and losses.
7. b) The amount of water in a reservoir — a stock is an accumulation measurable at a point in time.
8. c) Reducing the gain of the reinforcing loop through cognitive reappraisal.
9. b) A balancing loop driven by a byproduct of the system's own activity.
10. c) The delay between adjusting the tap and feeling the temperature change causes overshooting.
11. False. "Negative" refers to the direction of the feedback (opposing change), not its quality. Negative feedback is often highly beneficial (e.g., homeostasis).
12. True. Real systems almost always have multiple loops operating simultaneously.
13. False. Every real system has physical limits. The gain determines the tendency toward runaway, but growth always encounters constraints (resource depletion, physical limits, external intervention).
14. False. The oscillations are endogenous — caused by the delays inherent in the predator-prey feedback loops, not by external disturbances.
15. True.
16. False. The chapter's central argument is that feedback principles are substrate-independent and apply identically across engineered, biological, economic, and psychological systems.
17. True. This was a key insight of Jay Forrester and the system dynamics school.
18. False. In the Richardson model, each nation's buildup is a direct response to the other nation's buildup — the loop connects the two.
19. The four components are: (1) Sensor — something that measures the current state; (2) Reference signal — the desired state or target; (3) Comparator — computes the difference (error signal) between current state and target; (4) Actuator — acts on the system to reduce the error. For a central bank: (1) Sensor = economic data on inflation; (2) Reference = target inflation rate (~2%); (3) Comparator = analysis of whether inflation is above or below target; (4) Actuator = interest rate adjustments.
20. An arms race is a reinforcing feedback loop: Nation A builds weapons, Nation B perceives a threat and builds more, Nation A perceives B's buildup as threatening and builds even more. The gain of the loop is greater than one — each side's response exceeds what a purely defensive assessment would require, because each side is responding not just to the current threat but to the anticipated trajectory of the other's buildup. The result is escalation far beyond any rational assessment of military need, driven not by strategy but by the structure of the loop itself.
21. All four systems oscillate because of delay in the feedback loop. The system overcorrects because by the time the corrective action takes effect, the system has already moved past the set point. The correction then pushes the system past the set point in the other direction, and the cycle continues.
22. One example: CBT for anxiety disorders. The reinforcing loop: worry triggers physical symptoms, which are interpreted as evidence of danger, which increases worry. The gain mechanism: anxiety sensitivity — the tendency to interpret physical sensations as threatening. The intervention: cognitive reappraisal teaches the patient to interpret physical sensations as normal and non-threatening, reducing the gain of the interpretation step to less than one, converting the reinforcing loop into a self-correcting one.
23. A stock is an accumulation — something measurable at a point in time (water in a bathtub, money in an account, CO2 in the atmosphere). A flow is a rate of change — something measurable over an interval (water per minute, dollars per month, tons of CO2 per year). A common confusion: interpreting declining emissions growth rates (a change in the flow) as evidence that atmospheric CO2 (the stock) is decreasing. Even if the flow slows, the stock continues to rise as long as inflow exceeds outflow.
24. It is true in the sense that both systems share the same abstract structure (sensor, reference, comparator, actuator) and the same mathematical description (negative feedback with a set point). The dynamics — correction toward equilibrium, oscillation with delay — are genuinely identical. It might be an oversimplification in that the complexity of interacting loops in the immune system far exceeds that of a thermostat, and the immune system can modify its own reference signals (e.g., fever "resets" the set point), introducing dynamics that a simple thermostat model does not capture.
25. Answers will vary but should convey: Substrate independence means that feedback loop dynamics are determined by the structure of the loop (gain, delay, positive vs. negative), not by the physical material the system is made of. It is radical because it means knowledge gained in one domain (e.g., engineering) transfers directly to completely different domains (e.g., psychology, economics), and the transfer is not metaphorical but mathematically precise.