Positive feedback

Positive feedback, sometimes referred to as "cumulative causation", refers to situations where some effect causes more of itself. A system undergoing positive feedback is unstable, that is, it will tend to spiral out of control as the effect amplifies itself.

Technically, a system exhibiting positive feedback responds to perturbation acts to increase the magnitude of a perturbation. That is, "A produces more of B which in turn produces more of A". In contrast, a system that responds to the perturbation in the opposite direction is said to exhibit negative feedback. 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 usually not "positive" in the sense of being desirable. Positive refers to the direction of change rather than the desirability of the outcome. A negative feedback loop tends to reduce or inhibit or stabilise a process, while a positive feedback loop tends to expand or promote it and will often ultimately destabilise it.