PGTEST This page was automatically generated by NetLogo 1.3.1. Questions, problems? Contact

In order for this to work, this file, your model file (SerendipSegregation.nlogo), and the file NetLogoLite.jar must all be in the same directory. (You can copy NetLogoLite.jar from the directory where you installed NetLogo.)

On some systems, you can test the applet locally on your computer before uploading it to a web server. It doesn't work on all systems, though, so if it doesn't work from your hard drive, please try uploading it to a web server.

You don't need to include everything in this file in your page. If you want, you can just take the HTML code beginning with <applet> and ending with </applet>, and paste it into any HTML file you want. It's even OK to put multiple <applet> tags on a single page.

If NetLogoLite.jar and your model are in different directories, you must modify the archive= and value= lines in the HTML code to point to their actual locations. (For example, if you have multiple applets in different directories on the same web server, you may want to put a single copy of NetLogoLite.jar in one central place and change the archive= lines of all the HTML files to point to that one central copy. This will save disk space for you and download time for your user.)

created with NetLogo

view/download model file: SerendipSegregation.nlogo

This project models the behavior of two types of turtles in a mythical pond. The red turtles and green turtles get along with one another. But each turtle wants to make sure that it lives near some of "its own." That is, each red turtle wants to live near at least some red turtles, and each green turtle wants to live near at least some green turtles. The simulation shows how these individual preferences ripple through the pond, leading to large-scale patterns.

This project was inspired by Thomas Schelling's writings about social systems (such as housing patterns in cities).

Click the SETUP button to set up the turtles. There are equal numbers of red and green turtles. The turtles move around until there is at most one turtle on a patch. Click GO to start the simulation. If turtles don't have enough same-color neighbors, they jump to a nearby patch.

The NUMBER slider controls the total number of turtles. (It takes effect the next time you click SETUP.) The %-SIMILAR-WANTED slider controls the percentage of same-color turtles that each turtle wants among its neighbors. For example, if the slider is set at 30, each green turtle wants at least 30% of its neighbors to be green turtles.

The "Percent Similar" monitor shows the average percentage of same-color neighbors for each turtle. It starts at about 0.5, since each turtle starts (on average) with an equal number of red and green turtles as neighbors. The "Percent Unhappy" monitor shows the percent of turtles that have fewer same-color neighbors than they want (and thus want to move). Both monitors are also plotted.

When you execute SETUP, the red and green turtles are randomly distributed throughout the pond. But many turtles are "unhappy" since they don't have enough same-color neighbors. The unhappy turtles jump to new locations in the vicinity. But in the new locations, they might tip the balance of the local population, prompting other turtles to leave. If a few red turtles move into an area, the local green turtles might leave. But when the green turtles move to a new area, they might prompt red turtles to leave that area.

Over time, the number of unhappy turtles decreases. But the pond becomes more segregated, with clusters of red turtles and clusters of green turtles.

In the case where each turtle wants at least 30% same-color neighbors, the turtles end up with (on average) 70% same-color neighbors. So relatively small individual preferences can lead to significant overall segregation.

Try different values for %-SIMILAR-WANTED. How does the overall degree of segregation change?

If each turtle wants at least 40% same-color neighbors, what percentage (on average) do they end up with?

In the UPDATE-GLOBALS procedure, note the use of SUM, COUNT, VALUES-FROM, and WITH to compute the percentages displayed in the monitors and plots.

Schelling, T. (1978). Micromotives and Macrobehavior. New York: Norton.
Seee also a recent Atlantic article: Rauch, J. (2002). Seeing Around Corners; The Atlantic Monthly; April 2002;Volume 289, No. 4; 35-48.

To refer to this model in academic publications, please use: Wilensky, U. (1998). NetLogo Segregation model. Center for Connected Learning and Computer-Based Modeling, Northwestern University, Evanston, IL.