Segregation-New

Modifications of the Segregation Model:

The change in this model lies in the way in which turtles move. In the original version turtles who wish to move would pick a random heading and then jump between 0 and 9 spaces in that direction. Once there they would look around to make sure that another turtle wasn't already there. If the space was already occupied the turtle would randomly jump again and again until it hits an empty space. I found this to be a heavily unrealistic method of movement. The new model has a few significant changes.

The modified behavior is controlled with the JUMPING? switch. With the switch set to off, the model behaves as it did before the modifications.
Turtles can only move one space at a time. No more jumping.
Turtles first look for an empty space before they move. Turtles will not move to a space that is already occupied.
If there are no empty spaces next to a turtle who wishes to move that turtle will pick a random neighbor and switch places with it.
Turtles prefer to move into an empty space over swapping with a neighbor.
20% of the time that there is an empty space next to a turtle, the turtle will choose to swap with a neighbor instead of moving into that empty space.

created with NetLogo

view/download model file: Segregation-New.nlogo

WHAT IS IT? ------------ 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). HOW TO USE IT -------------- 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 JUMPING? switch controls whether or not turtles are allowed to jump multiple spaces in a single move. With JUMPING? turned on the model works exactly the same as the original Segregation model. With JUMPING? turned off the turtles can only move one space at a time. A turtle is only allowed to move into an unoccupied space or swap locations with an adjacent neighbor. 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. THINGS TO NOTICE ---------------- 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. THINGS TO TRY ------------ 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? NETLOGO FEATURES ---------------- 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. In FIND-NEW-SPOT notice the use of WITHOUT-INTERRUPTION. This is critical to correct behavior of turtles during movement. CREDITS AND REFERENCES ---------------------- 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. http://www.theatlantic.com/issues/2002/04/rauch.htm To refer to this model in academic publications, please use: Wilensky, U. (1998). NetLogo Segregation model. http://ccl.northwestern.edu/netlogo/models/Segregation. Center for Connected Learning and Computer-Based Modeling, Northwestern University, Evanston, IL.