Nobody here wants segregation. Each agent is happy as long as merely a third of its neighbours are like it — a mild, tolerant wish. Slide the rounds and watch that gentle preference, iterated, tip a mixed city into stark blocks. The pattern is nobody’s intent; it’s the between.
Two colours on a grid with some empty cells. An agent is unhappy only if fewer than 33% of its occupied neighbours share its colour; unhappy agents hop to a random empty cell. The instrument runs this deterministically (seeded) and measures the segregation index = the average fraction of same-colour neighbours. A fail-loud self-check throws unless that index RISES from its mixed start and ends far above the 33% threshold — Schelling’s 1971 point: micro-tolerance still yields macro-segregation.
One seed, one threshold, one grid — the specific numbers are illustrative; the mechanism (mild local preference → strong global sorting) is the robust, reproduced result. It is a model of a dynamic, not a claim about any real population.