Daniel's NetLogo Projects Page
Segregation with multiple races and "stubborness"
Two additions have been made to the original segregation model:
-- control over the number of races, up to 5
-- "stubborn" turtles (turtles which never move)
The number of races can be adjusted from 2 to 5 using the NUMBER-OF-RACES slider. The different
colors available are red, green, blue, magenta, and white. Individual turtles still follow the
model's original rules; they prefer neighbors of their own color in a minimum proportion as given
by the %-SIMILAR-WANTED slider.
The number of stubborn turtles is chosen by the corresponding slider. These turtles are chosen as
stubborn randomly during intialization. The designation is static during a model's running.
Stubborn turtles are highlighted by a YELLOW patch. They are rather obstinate turtles, entrenched
in their ways and hence never move. Their race affects other turtles normally. Predictably, similarly
colored clusters tend to form around stubborn turtles.
created with NetLogo
view/download model file: segregation-multiracial.nlogo
WHAT IS IT?
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This project models the behavior of multiple (from two to five) types of turtles in a mythical pond. The red, green, blue, magenta, and white 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, etc. The simulation shows how these individual preferences ripple through the pond, leading to large-scale patterns.
Additionally, a number of turtles can be termed "stubborn." These randomly selected turtles still have racial characteristics which affect their neighbors, however they obstinately refuse to move, regardless of surroundings.
This project was inspired by Thomas Schelling's writings about social systems (such as housing patterns in cities).
HOW TO USE IT
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Click the SETUP button to set up the turtles. The different races are always represented equally and randomly. 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.
There are two additional sliders. The NUMBER-OF-RACES slider selects from 2 to 5 races to be intialized. The NUMBER-OF-STUBBORN slider selects the number of "stubborn" turtles to be randomly selected. The two sliders' default values of 2 and 0, respectively, retain the original model's characteristics.
The "Percent Similar" monitor shows the average percentage of same-color neighbors for each turtle. It starts at about 1/n (where 'n' is the number of races), since each turtle starts (on average) with an even mix of different colored 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
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When you execute SETUP, the red and green (and possibly blue, magenta, and white) 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 same colored turtles emerging.
With the default settings of 2000 turtles, 2 races, no "stubborns," and where each turtle wants at least 30% same-color neighbors, the turtles end up with (on average) 71% same-color neighbors. So relatively small individual preferences can lead to significant overall segregation.
Surprisingly, adding races does not seem to change these numbers much. Increasing from 2 to 5 races, the final "percent-similar" count is only a few percentage points higher -- around 74% on average.
Predictably, using stubborn turtles controls the grouping patterns of the races. Same color clusters emerge around stubborn turtles.
THINGS TO TRY
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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?
Can you find a combination of settings in which adjusting the number of races significantly impacts the results? In which adjusting the number of "stubborns" shows an impact?
NETLOGO FEATURES
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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. When selecting the stubborns, this models uses the RANDOM-ONE-OF primitive.
CREDITS AND REFERENCES
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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.