Population Model
Posted: Sun Jan 23, 2022 5:03 pm
Year 2022 marks the 50th anniversary of the introduction of the Population Model, the most popular model in system dynamics. The model was first published in a very popular book at that time, “The Limits to Growth” (1972) by Meadows. Book was a success but nobody noticed that the Population Model was incorrect.
The intuitive approach to system dynamics taken by the authors led them to believe that positive (reinforcing) feedback always creates growth and decline is the results of negative (balancing) feedback. At first glance this approach may look logical but this is not how reinforcing and balancing feedback work and how to distinguish one from the other. The key differentiating criteria between them is not an odd or even number of negative signs (because development of any model is wide open to interpretations) but the convergence (or lack of it) to specific goal. The reinforcing feedback loops have no specific goals to reach so they drift from plus to minus infinity (or zero). The balancing feedback loops however always have specific goals (objective) to achieve and maintain.
The authors of the book overlooked the fact that deaths are also a phenomena with reinforcing (positive) feedback. Births could increase population to infinity and deaths could decline population to zero (superposition helps to see that). The claimed “balancing effect” of the two feedback loops of opposite polarity is actually a result of two reinforcing feedback loops counterbalancing each other (working against each other). This is not a stable system.
Note: This observation by no means undermines the usefulness of Vensim program, which has permanent place in my toolbox.
The intuitive approach to system dynamics taken by the authors led them to believe that positive (reinforcing) feedback always creates growth and decline is the results of negative (balancing) feedback. At first glance this approach may look logical but this is not how reinforcing and balancing feedback work and how to distinguish one from the other. The key differentiating criteria between them is not an odd or even number of negative signs (because development of any model is wide open to interpretations) but the convergence (or lack of it) to specific goal. The reinforcing feedback loops have no specific goals to reach so they drift from plus to minus infinity (or zero). The balancing feedback loops however always have specific goals (objective) to achieve and maintain.
The authors of the book overlooked the fact that deaths are also a phenomena with reinforcing (positive) feedback. Births could increase population to infinity and deaths could decline population to zero (superposition helps to see that). The claimed “balancing effect” of the two feedback loops of opposite polarity is actually a result of two reinforcing feedback loops counterbalancing each other (working against each other). This is not a stable system.
Note: This observation by no means undermines the usefulness of Vensim program, which has permanent place in my toolbox.