Schelling’s Segregation Model

Outline

In 1969, Thomas C. Schelling developed a simple but striking model of racial segregation [Sch69]

His model studies the dynamics of racially mixed neighborhoods

Like much of Schelling’s work, the model shows how local interactions can lead to surprising aggregate structure

In particular, it shows that relatively mild preference for neighbors of similar race can lead in aggregate to the collapse of mixed neighborhoods, and high levels of segregation

In recognition of this and other research, Schelling was awarded the 2005 Nobel Prize in Economic Sciences (joint with Robert Aumann)

In this lecture we (in fact you) will build and run a version of Schelling’s model

The Model

We will cover a variation of Schelling’s model that is easy to program and captures the main idea

Set Up

Suppose we have two types of people: orange people and green people

For the purpose of this lecture, we will assume there are 250 of each type

These agents all live on a single unit square

The location of an agent is just a point \((x, y)\), where \(0 < x, y < 1\)

Preferences

We will say that an agent is happy if half or more of her 10 nearest neighbors are of the same type

Here ‘nearest’ is in terms of Euclidean distance

An agent who is not happy is called unhappy

An important point here is that agents are not averse to living in mixed areas

They are perfectly happy if half their neighbors are of the other color

Behavior

Initially, agents are mixed together (integrated)

In particular, the initial location of each agent is an independent draw from a bivariate uniform distribution on \(S = (0, 1)^2\)

Now, cycling through the set of all agents, each agent is now given the chance to stay or move

We assume that each agent will stay put if they are happy and move if unhappy

The algorithm for moving is as follows

  1. Draw a random location in \(S\)
  2. If happy at new location, move there
  3. Else, go to step 1

In this way, we cycle continuously through the agents, moving as required

We continue to cycle until no one wishes to move

Results

Let’s have a look at the results we got when we coded and ran this model

As discussed above, agents are initially mixed randomly together

../_images/schelling_fig1.png

But after several cycles they become segregated into distinct regions

../_images/schelling_fig2.png
../_images/schelling_fig3.png
../_images/schelling_fig4.png

In this instance, the program terminated after 4 cycles through the set of agents, indicating that all agents had reached a state of happiness

What is striking about the pictures is how rapidly racial integration breaks down

This is despite the fact that people in the model don’t actually mind living mixed with the other type

Even with these preferences, the outcome is a high degree of segregation

Exercises

Rather than show you the program that generated these figures, we’ll now ask you to write your own version

You can see our program at the end, when you look at the solution

Exercise 1

Implement and run this simulation for yourself

Consider the following structure for your program

Agents are modeled as objects

(Have a look at this lecture if you’ve forgotten how to build your own objects)

Here’s an indication of how they might look

* Data:

    * type (green or orange)
    * location

* Methods:

    * Determine whether happy or not given locations of other agents

    * If not happy, move

        * find a new location where happy

And here’s some pseudocode for the main loop

while agents are still moving
    for agent in agents
        give agent the opportunity to move
    end
end

Use 250 agents of each type