Why Four? Working Memory Capacity as Evolutionary Equilibrium in a Biomemetic Architecture

Why can humans hold approximately four items in working memory? Cowan's "magic number" has been explained mechanistically but never derived from evolutionary first principles. Using the Biomemetic Complex (BMC) architecture with heritable WM capacity and metabolic brain cost, we show that delayed-reward foraging creates a k-dependent fitness advantage (2–4× harvest rate for k=4 vs k=2) that stabilizes population mean at k≈4.4 — Cowan's 4±1 range. The attractor is confirmed by convergence from both directions: upward from mean 4.0 and downward from 5.0. A robustness–differential tradeoff constrains planning evolution: architectural innovations that make planning universally reliable simultaneously eliminate the selection gradient.