Sunday, 29 December 2013

How Mild Preferences Can Lead To All-Out Segregation

I once wrote a Blog post on how riots actually start. Today I want to turn to someone else who has found the answer to a similar issue: Why do we have major racial/sexual/religious segregation when at an individual micro level the motives and prejudices are much less distinguishable than at the macro level?

Nobel Prize winning economist Thomas Schelling found out why; he showed that even a tiny amount of discrimination can lead to an almost total segregation. Schelling used coins on graph paper (or, if you prefer, pieces on a chess board for simplicity) to exhibit this phenomenon; he placed pennies and nickels in different patterns on the board to represent black people and white people. He showed that if you remove 20 random coins and add 5 random coins back again you have a situation ripe for racial segregation (see figure (a) above).

Suppose that each coin had a rule - it no longer wished to be positioned in a place on the graph in which it was outnumbered by other non-similar coins by more than 2 to 1. Schelling showed that by moving each coin away, consistent with that desired rule, that even from such mild preferences it creates a major segregation (see figure (b) above).

Rather like when Charles Booth's infamous poverty map showed how London's once poor estates are largely the ones that have remained poor, Thomas Schelling put in a lot of mathematical groundwork to show that, just like the coin model, tension-filled segregation does not have to be incubated by, or even gestated by, extreme prejudice and ill-feeling - even mild prejudices do often eventuate in all-out segregation.

Of course, Schelling's model was very much centred on actual neighbourhoods with geographical proximity. But nowadays, with online global connectivity, I suspect those tensions are not confined to the neighbourhood or local estate - they are very much part of a wider dynamical tension that stratifies into tribal groups comprising many people who've never even met each other.

* Photo courtesy of