System Dynamics notation:
System dynamics diagrams are designed to help overcome the frequent tendency to default to linear assumptions about causation. These diagrams, sometimes known as causal loop diagrams, do this by helping us trace more complex patterns of causation: circles and webs. Causal loop diagrams are arranged so as to communicate complex patterns better by eliminating clutter and overlapping arrows and by achieving some measure of symmetry so that it is easy for the eye to follow paths of ‘causation,’ such as around loops or across webs. Thus feedback loops may be clockwise or counterclockwise for stylistic reasons, but the influence of the loop on dynamics is mostly related to whether it is a reinforcing or balancing loop by itself and as to how it is structurally linked to other loops that may counter or reinforce its actions. Following the System Dynamics protocol, arrows denote a relation between two variables. A (+) polarity indicates that both change in the same direction, e.g., if one increases the other tends to increase as well. A (-) polarity indicates that when one changes in value the other tends to change in the opposite direction. Feedback loops are distinguished by the letter R (reinforcing) and B (balancing). Change will continue in the same direction by the time a circuit is made all the way around a reinforcing feedback loop. That means that increase in one variable in a reinforcing feedback loop will end up increasing even further by the chain of interactions going all the way around the loop, and a decrease would also be augmented by completing the circuit. By contrast, the direction of change will be reversed by completing a circuit around a Balancing Feedback loop. So an increase in one variable will eventually be translated into a decrease of that variable when the chain of interactions goes all the way back to the point of origin. For example, in Fig. 7,the loop Bd1 is balancing because the polarities + - + + end up reversing the initial change by the end of the loop. Try reading the polarities as words, then the series becomes: same(+), different(-), same(+), same(+).