Mutual causal relationships may be defined between more than two elements. Let us look at the following diagram. The arrows indicate the direction of influences. + indicates that the changes occur in the same direction, but not necessarily positively. For example, the + between G and indicates that an increase in the amount of garbage per area causes an increase in the number of bacteria per area. But, at the same time, it indicates that a decrease in the amount of garbage per area causes a decrease in the number of bacteria per area. The — between S and B indicates that an increase in sanitation facilities causes a decrease in the number of bacteria per area. But, at the same time, it indicates that a decrease in sanitation facilities causes an increase in the number of bacteria per area.
As may be noticed, some of the arrows form loops. For example, there is a loop of arrows from P to M, M to C, and C back to M. A loop indicates mutual causal relationships. In a loop, the influence of an element comes back to itself through other elements. For example, in the loop of P-M-C-P, an increase in the number of people causes an increase in modernization, which in turn increases migration to the city, which in turn increases the number of people in the city. In short, an increase in population causes a further increase in population through modernization and migration. On the other hand, a decrease in population causes a decrease in modernization, which in turn causes a decrease in migration, which in turn decreases population. In short, a decrease in population causes a further decrease in population through decreased modernization and decreased migration. Whatever the change, either an increase or a decrease, amplifies itself. This is so when we take population as our criterion. But the same is true if we take modernization as a criterion: an increase in modernization causes a further increase in modernization through migration and population increase; and a decrease in modernization causes a further decrease in modernization through decreased migration and decreased population. The same holds true if we take the migration as the criterion.
In a loop, therefore, each element has an influence on all other elements either directly or indirectly, and each element influences itself through other elements. There is no hierarchical causal priority in any of the elements. It is in this sense that we understand the mutual causal relationships.