English: Feedback Loops in a System Dynamics Model

The world that analysts attempt to simulate is more complicated than that, as this figure suggests.

The bold loop in the figure is the classic cybernetic loop, striving to bring the state of the system toward some set goal. However, unstated goals often intervene and unintended effects are triggered. The system changes from its own forces, and all sorts of effects feed back to alter the actions of the actors. (In a system dynamics perspective, there are no “side” effects, only effects.) Moreover, complex systems have many actors, each with personal or organizational goals, so this structure is repeated countless times in real systems. The result is that actions one group takes to reach toward its goals disturb the system and prompt other groups to implement counteraction, striving to reassert the status quo or lead to a different status quo. In contrast, the simpler cause-and-effect behavior can result when these factors are held constant. Thus, a system often will compensate for changes and weaken or even negate them, much as a price cut can stimulate competitive forces that negate its original goal of increasing sales. This phenomenon is referred to as policy resistance.