Even a slight preference for homophily results in excessive segregation

This was economist Thomas Schelling’s insight way back in 1969 — just one of many examples of “unknown knowledge” that exists in the world today. His Spatial Segregation Model takes a few simple premises and shows that a set of quite tolerant people, who genuinely prefer to live in a diverse neighborhood in terms of race, income, and other factors, nevertheless end up self-segregating into clusters of like individuals — as follows:

Slight preference for homophily: 30%

We set up a fairly dense environment with a low preference for similarity — people are quite tolerant and are only looking to have 30% of their near neighbors be similar to them:

But when we run the simulation, we end up with an equilibrium state where individuals are surrounded by 75.2% similar neighbors:

If we run the spatial segregation model with a 50% preference for similar neighbors, the outcome is even more stark: the agents achieve equilibrium at a whopping 87.7% similarity:

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