​This model attempts to understand the behavior of average lifetime of companies in the S&P500 index. The reference mode for the model is a graph available at this link: https://static-cdn.blinkist.com/ebooks/Blinkracy-Blinkist.pdf (page 5) which was discussed in the System Thinking World Discussion forum.

Mergers & Acquisitions can be one of the reasons for older companies to be replaced with newer companies in the Index. With M&A of older companies, the empty slots are taken over by newer companies. However, overtime, these new companies themselves become old. With steady M&A, the stock of older companies decreases and stock of newer companies increases. The result is that average age of the companies in the S&P Index decreases.

The oscillations in the diagram, according to me, is due to oscillations in the M&A activity.

There are two negative feedback loops in the model. (1) As stock of new companies increases, the number of companies getting older increases which in turn decreases the stock. (2) As M&A increases, stock of older companies decreases which in turn decreases M&A activities.

Limits of the model

The model does not consider factors other than M&A in the increase in number of new companies in the Index. New companies themselves may have exceptional performance which will result in their inclusion in the Index. Changes in technology for example Information Technology can usher in new companies.

Assumptions

1. It is assumed that M&A results in addition of new companies to the Index. There could be other older companies too, which given the opportunity, can move into the Index. Emergence of new technologies brings in new companies.

This model represents a repair contract to fix a group of houses with unresolved construction defects.

Causal loop structure of system dynamics models of the business cycle and the Kondratieff long wave from Gene Bellinger's AI gemini prompts mc and mr Jan2026 From MIT Sloan school work esp Forrester and Sterman

WIP Overview model structures of Khalid Saeed's 2014 WPI paper Jay Forrester’s Disruptive Models of Economic Behavior  See also General SD and Macroeconomics CLDs IM-168865

A simulation of burnout based on workload and amount of sleep.

Exercise: Build a stock and flow diagram with Insight Maker | LinkedIn Learning

Introduction to system dynamics | LinkedIn Learning

Modelo Predador Presas

Based on a dialogue on the System Dynamics mailing list regarding the current level of acceptance of System Dynamics after it has been promoted for over 70 years I dredged up the following set of influences as a thought exercise. This is an example of a Drifting Goals Archetype.

@LinkedIn, Twitter, YouTube

This is a stage-structured population growth model of the Loggerhead Turtle based on the model developed by Crouse et al. (1987). 

Australian health care challenges, inspired by Gene Bellinger's AI gemini causal loop modelling of US healthcare posted to substack in Jan2026

From Jay Forrester 1988 killian lectures youtube video describing system dynamics at MIT. For more detailed biography See Jay Forrester memorial webpage For MIT HIstory see IM-184930 For Applications se IM-185462

Causal loop structure of Steve Keen's video Why 'Balancing The Budget' Will Actually Crash The Economy generated by my Gemini interaction  Feb2026 using Gene Bellinger's AI prompts

Daniel Stanton's LinedIn Learning Course:

The System Dynamic Model represents the Covid19 cases in Brgy. Sicsican, Puerto Princesa City as of May 27,2022. 

Causal loop diagram of the John Baez 2023 article Compositional Modeling with Stock-Flow Diagrams describing work with Nate Osgood on System dynamics epidemiology models based on my Gemini interaction using Gene Bellinger's AI prompts