Friday Quiz: Feedback Loops, Positive and Negative   (February 20, 2009)

What is the difference between positve and negative feedback loops?

A: Positive feedback, sometimes referred to as "cumulative causation", is a feedback loop system in which the system responds to perturbation in the same direction as the perturbation.

In contrast, a system that responds to the perturbation in the opposite direction is called a negative feedback system. These concepts were first recognized as broadly applicable by Norbert Wiener in his 1948 work on cybernetics.

The effect of a positive feedback loop is not necessarily "positive" in the sense of being desirable. The name refers to the nature of change rather than the desirability of the outcome. The negative feedback loop tends to slow down a process, while the positive feedback loop tends to speed it up.

A system in equilibrium in which there is positive feedback to any change in its current state is said to be in an unstable equilibrium, whereas it is possible for one with negative feedback to be in a stable equilibrium.

The end result of a positive feedback is often amplifying and "explosive", i.e. a small perturbation results in big changes. This feedback, in turn, will drive the system further away from its original set point, thus amplifying the original perturbation signal, and eventually become explosive because the amplification often grows exponentially (with the first order positive feedback), or even hyperbolically (with the second order positive feedback).

A simple and practical example of negative feedback is a thermostat. When the temperature in a heated room reaches a certain upper limit the room heating is switched off so that the temperature begins to fall. When the temperature drops to a lower limit, the heating is switched on again. Provided the limits are close to each other, a steady room temperature is maintained. The same applies to a cooling system, such as an air conditioner, a refrigerator, or a freezer.

Some biological systems exhibit negative feedback such as the baroreflex in blood pressure regulation and erythropoiesis. Many biological process (e.g., in the human anatomy) use negative feedback. Examples of this are numerous, from the regulating of body temperature, to the regulating of blood glucose levels.

A kind and astute reader corrected my confusion on positive and negative feedback a few years ago. In essence, the question posed by the global financial meltdown could be phrased: which feedback loops will dominate the process? If positive feedback loops predominate, then we get the doomsday runaway-train/nuclear fission analogy in which the entire global financial system implodes and life becomes nasty, brutish and short for millions, if not billions.

Alternatively, if negative feedback loops predominate, then the downtrends currently in place will be slowly modified until a new equilibrium is established.

It seems many see only positive feedback loops in play, but I see numerous negative feedback loops in play as well. For instance, as gasoline prices rose and the economy stumbled, gasoline use fell dramatically in the U.S. This is a negative feedback loop. The government throwing trillions of borrowed dollars at "the problem" is also a negative feedback loop. Citizens demanding an end to the bank bailouts (see link in right sidebar) is also a negative feedback loop.

Panic tends to be a positive feedback, reasoned action based on practical gleanings from history tend to be a negative feedback loop.

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