To demonstrate how a causal loop works, we can use an age-old problem. “Why did the chicken cross the road?”  Because Systems Thinking is concerned with causal relationships, we are more concerned with the dynamic interaction between chickens and road crossings, rather than the chicken’s motivation.

Our CLD would look like this.

The two arrows indicate causal connections. The letters at the arrowheads indicate the behaviour of the variables, in this case road crossings and chickens.

The letter S at the head of the arrow connecting chickens and road crossings indicates that these two variables move in the same direction. As the number of chickens increases, the number of road crossings goes up. Similarly, as the number of chickens goes down so too does the number of road crossings.

The letter O at the hint of the arrow connecting road crossings and chickens indicates that these two variables move in the opposite direction. As the number of road crossings increases, the number of chickens goes down. Similarly as the number of road crossings goes down, the number of chickens goes up.

The reason that the number of chickens goes up is shown in the second half of the diagram that demonstrates the dynamic relationship between chickens and eggs.

In this part of the diagram, the number of chickens goes up, the number of chickens goes up and then the number of eggs goes up again.

This dynamic shows an important aspect of the way we read CLDs. We can trace the causal connections through the diagram to understand the impact that any one part of the system may have on another part. In this case we know that if the number of eggs goes up, the number of road crossings will increase. This is called “walking through” a causal loop diagram. As we do this, the story that we can tell must be logical and plausible. If this is the case, then it is highly likely that the causal relationships are correct.

The other important principle is the interrelationship between the two loops. The road crossings/chickens loop is called a balancing loop because the number of road crossings serves to bring the number of chickens into balance. The chickens/eggs loop is reinforcing loop, like all population loops, with the two variables served to increase each other’s numbers.

In determining the behaviour of a system is necessary to understand which group will dominate. In this case, it will be the chickens/eggs loop. So overall the trick and population will be increasing.

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