Fig. 2. Labelling conventions for causal links. The arrows indicate that a change in variable A causes a change in variable B. Each link is assigned a polarity. The polarity of the causal link between A and B is said to be positive when an increase/decrease in A causes B to increase/decrease above/below what it otherwise would have been. A causal link is negative when an increase/decrease in A causes B to decrease/increase below/above what it otherwise would have been. A short line indicates that there is a delay between a change in A and the corresponding change in B.