Definition
System Dynamics is a top-down modeling approach used to understand and influence how complex systems change through time. It focuses on the macro-level representation of a system, emphasizing the interdependence of actors, events, and variables.
Why It Matters
It exposes the counter-intuitive nature of complex systems, where cause and effect are often separated in time and space. Without understanding feedback loops and delays, policy interventions often produce the exact opposite of their intended results (policy resistance).
Core Concepts
- Causal Loop Diagrams: Diagrams that describe how system variables relate from a cause-and-effect standpoint.
- Reinforcing Loops (+): Positive feedback that leads to exponential growth or decay.
- Balancing Loops (-): Negative feedback that seeks stability or equilibrium.
- Stock and Flow Models:
- Stocks: Accumulation points that measure the amount of a variable at any given time (e.g., population).
- Flows: Inputs and outputs that represent the rate of change of the stock.
# Simple Stock and Flow Logic
# New_Stock = Old_Stock + (Inflow - Outflow) * dt
population = 1000
birth_rate = 0.02
death_rate = 0.01
# Update in one time step
inflow = population * birth_rate
outflow = population * death_rate
population += (inflow - outflow)- Propensity Factors: Intangible variables (e.g., culture, religion, policy) that influence the rates of change in social models.
- Founder: Professor Jay Forrester (MIT).