Verify the driving variable is actually coupled before claiming an emergent effect

Before writing a multi-tick sim to prove cities grow differently when trade reroutes, I grepped for every write to the growth-driving variable (prosperity) and found it was set only by static grants + a random drift — trade never touched it. So the effect could not emerge; I built the missing coupling first (arrival lifts prosperity; prosperity mean-reverts to a baseline). Lesson: when a sim will assert a downstream emergent effect, grep for what writes the causal state var first — if nothing links cause to effect, build that coupling before the sim, or you are asserting something the engine cannot produce. Second lesson: the signal turned out arrival-FREQUENCY driven, so the boom was a transient leaving a lasting population gap, not a steady-state divergence — characterize transient vs steady-state and frame the claim to match rather than overclaim.