◆ The PatternWhen a system’s output feeds back into its input, small nudges can cascade into enormous change
// When a system’s output feeds back into its input, small nudges can cascade into enormous change
A savings account grows slowly at first. Interest earns interest, which earns more interest. After a few years the line barely looks bent. After a few decades it curves sharply upward. The rules never changed. Only time passed. This is compounding: the simplest and most powerful feedback loop.
But exponential growth always meets a wall: resources run out, competition arrives, the body builds immunity. The result is an S-curve: slow start, explosive middle, plateau at the top. Population growth, technology adoption and epidemics often follow this shape. The feedback loop is the engine; the ceiling is the brake.
Negative feedback works in reverse: the output damps the system back toward equilibrium. A thermostat. A predator-prey cycle. The price mechanism in a market. Without negative feedback, every small perturbation would spiral forever.
Growth rate20%/yr
Exponential growth hitting a ceiling: the S-curve
What to rememberFind the loop, meaning what feeds back into what. The loop decides where a system ends up, far more than the starting point does.
References
[1] Meadows, D. H. (2008). Thinking in Systems: A Primer. Chelsea Green Publishing.
[2] Strogatz, S. (2003). Sync: How Order Emerges From Chaos in the Universe, Nature, and Daily Life. Hyperion.
[3] Verhulst, P.-F. (1838). Notice sur la loi que la population suit dans son accroissement. Correspondance Mathématique et Physique, 10, 113–121.
Pattern bridge: The distribution shape topic shows what happens when feedback loops generate extreme outcomes, and moving averages are a practical negative-feedback tool. Where a loop runs into the slowest step in a system, see The Bottleneck.