Open systems (ecological, biological, social) contain many types of regulatory circuits, among which positive and negative feedbacks. Positive and negative does not mean desirable or not. The negative feedback loop tends to slow down a process, while the positive feedback loop tends to accelerate it.
When a change of variable is occuring in a system, the system responds. In positive feedback the response of the system is to change that variable even more in the same direction. This has a de-stabilizing effect, so does not result in homeostasis. In some cases (if not controlled by negative feedback), a positive feedback loop can run out of control, and can result in the collapse of the system.
Positive feedback is used in certain situations where rapid change is desirable.
See also
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