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January 22, 1998


Segregation Simulation FAQ

By PETER WAYNER


Q: How many people are in the simulation?


A: The computer inserts 300 blue people and 300 red people.


Q: How do the people move?


A: Every turn, the computer selects someone at random. That person then begins evaluating new empty spaces at random by counting the people with the same color around the space. If the number passes the threshold, then they move in. Otherwise, the person chooses a new location and repeats the process.


Q: This goes on forever?


A: Until you stop the simulation or change the parameters.


Q: How are the neighbors counted?


A: There are two methods. If you set the radius of a "neighborhood" to 1, then only the eight adjoining squares are examined. The number of similarly colored people are counted and checked against the threshold to see if the new spot is acceptable.

If you set the radius of the neighborhood to 2, then all spots within two units are checked. These results are weighted according to the distance from the square in question. The immediately adjacent spots are worth one unit, but those that are two spots away are worth only one half.

Here are the weightings for a neighborhood set to 2:

.35 -- .44 -- .5 -- .44 --.35
.44 -- .70 -- 1.0 -- .70 -- .44
.5 -- 1.0 -- xxx--- 1.0 -- .5
.44 -- .70 -- 1.0 -- .70 -- .44
.35 -- .44 -- .5 -- .44 --.35

If all of the 24 adjacent squares are filled, then the total weighting is 13.72.


Q: What about a position like this with the empty square in the center marked by a question mark?

BxRRx
BxxxR
BB?BB
RxRxR
RRxxR


A: If the radius of a neighborhood is set to be 1, then a blue person would find two neighbors and a red person would find one. If the threshold was set to be 2, then a blue person could move to this square but a red one would find it unacceptable.

If the radius is 2, then a red person would look around and find a weighted score of 4.40 and a blue person would find a weighted score of 3.79. If the threshold is set to be four, then a red person could move in but a blue one couldn't.


Q: Which method is more accurate?


A: Neither is more accurate. They're both just different ways of trying to create a simple rule about how people think. Some people may only look at immediate neighbors. Others may look over the hedges. Still others may not care at all. A more sophisticated simulation would look at mixtures of populations.


Q: How do the numbers affect the answer?


A: If the radius is 1, then even a threshold of 1 quickly leads to heavy segregation. That is, if each person requires just one immediate neighbor of the same group, then the model quickly becomes segregated. Higher thresholds make this happen even faster.

If the radius is 2, then a threshold of 3 is the turning point. If the threshold is 2, then the board remains fairly random. If people start requiring that at least three of their neighbors are of the same group, then the segregation will emerge in the simulation.


Q: What are "closed positions"?


A: These are spots where no one can live. They might simulate a freeway, a large industrial complex or some other place where people don't live. You can draw the spots on the board to watch how they affect the change.

Closed positions don't count as anyone's neighbor. That is, neither a red nor a blue person will find any additional comfort living next to a closed position.

One of the most illustrative experiments is to divide the board in half by a thin line simulating a freeway. Although people from both groups can and do continue to live on both sides, the line frequently becomes a color barrier as well.


Q: What are the problems with this simulation?


A: All people are different and make their decisions about where they live based upon a variety of criteria. Some may weigh race heavily while others may care more about commuting distance to work. This simulation only looks at groups and ignores other influence. Also, it treats everyone the same. Each person in the simulation behaves in exactly the same manner.


Q: Where are some good places to turn for more information?


A: Thomas Schelling did some of the original work in the area. The book Growing Artificial Societies: Social Science from the Bottom Up by Joshua Epstein and Robert Axtell is also a good summary of some of the latest work applying simulations to social phenomenon. Many researchers at the Santa Fe Institute also study the realm.

http://www.brook.edu/sugarscape/

http://www.santafe.edu/


Peter Wayner at pwayner@nytimes.com welcomes your comments and suggestions.




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