Definition
Compound interest of behavior is the exponential effect of small habits accumulating over time, where minor daily actions yield outsized results — for better or worse.
Just as money grows through compounding returns, behavior compounds through repetition. The result is rarely visible in any single day and unmistakable across years.
How it works
Compounding has three properties that make it counterintuitive: it is multiplicative rather than additive, it is slow at first then sudden, and it works in both directions.
Multiplicative means each unit of effort interacts with what came before. Today's workout adds strength to yesterday's strength; today's article reinforces last week's knowledge. The base grows, and growth scales on the base.
Slow at first means the early returns are emotionally underwhelming. You eat a salad and the scale does not move. You read for ten minutes and you do not feel smarter. The investment is real but the dividend has not yet capitalized. Most people quit here.
Bidirectional means neglect compounds too. Skipped workouts, deferred sleep, postponed practice — each is a tiny withdrawal whose interest accrues silently. You wake up one day to find the deficit is large.