Feedback Loop

A feedback loop is system structure that causes output from one node to eventually influence input to that node. For example, the work output of a population can increase the goods and services available to that population, which can increase the average life expectancy, which can increase the population, which can increase the work output still more, and the loop starts all over again. Using system dynamics notation, this feedback loop would look like the Population Growth loop shown.

While this concept may seem simple and obvious, that is only because we have started with a simple model with only one loop and 4 variables. Few real world problems are as simple. For example, The Dueling Loops of the Political Powerplace model has 43 variables, 3 main loops, about 5 additional loops, and 4 stocks. It is a simplification of a larger model that has 123 variables, 11 stocks, and countless loops. Its construction took a single person several years. Another example is the World3 model in Limits to Growth, which contains about 300 variables and 20 stocks. It took 17 researchers 2 years to construct the model.

Understanding the behavior of difficult complex social system problems well enough to even begin to hypothesize a realistic solution, with a high probability of working the first time, is impossible without understanding the feedback loops involved.