Complex systems around us

Mechanism for residential segregation

Gather? Separate? Crowd together? Avoid others?

 Humans were originally creatures who formed groups in which to live, so there may be a “like attracts like” tendency for similar individuals to gather to reside in the same place. Conversely, differences of some kind may result in a tendency to live separately from others to avoid mutual contact. How should we try to understand this phenomenon of residential segmentation among humans?

 Sometimes we classify those around us into family/friends or “us” on the one hand, and outsiders or “them” on the other hand, gathering around the former and excluding or opposing the latter. This may lead to people living apart from each other.

One classic piece of research that attempts to resolve this problem head-on is the “segregation model.” This reveals the mechanisms by which the behavior of each individual is linked to the whole (⇒Reference to segregation model). The outcomes were surprising. Since then, various kinds of research have been developed.


 Another mechanism that causes phenomena similar to residential segregation/differentiation is “territory.” With individuals and populations as the units, humans rope off areas and wolves perform marking to designate their territory, thus asserting exclusive property and usage rights. When the assertions of one side are not recognized by the other, a territorial dispute ensues. Where it differs from residential segregation/differentiation, in which like individuals group together and avoid those who are different, is that designating territory has the outward-facing effect of excluding outsiders.

Modern-day states are defined as mutually exclusive areas by means of international borders, which are extremely sophisticated territorial markings. In order to cross international borders, we need passports and visas, and must submit to immigration control, and pass safely through customs and quarantine. Territorial disputes between different states are also known as border disputes. The single term “territorial dispute” in English is represented by two separate terms in Japanese.

Territory represents a space in which the owners and users can look for food and reproduce. In that sense, if territorial disputes can be limited to the areas around borders (frontiers), it is possible for creatures that maintain territories to coexist. The ayu, or sweetfish, derives its food from the moss on riverbeds, but it is known to create territories. By leveraging this aspect of the fish’s character, it becomes possible to use a kind of “angling by decoy.” However, ayu only assert their territories up to a certain level of population density, and when densities become high enough they seem to stop asserting their territories, form schools, and display patrolling behavior.


 In the Old City of Jerusalem, followers of the Jewish faith, Islam (Muslims) and Christianity (Catholicism and Eastern Church sects) practice residential segregation in a mosaic pattern, and have developed to form one local community overall. Examples of various communities with different cultures gathering together in this way to form local communities are by no means rare. However, there are also many examples in large cities of residential segregation, where even as people of the same culture gather together, they avoid contact with people of different cultures, or try to exclude them (⇒ Reference to residential segregation/differentiation).

 In the United States of America, prejudice and discrimination have led to residential segregation, particularly between lighter-skinned people who immigrated from Europe and those with darker skin who were brought from Africa as slaves. In addition, even among immigrants from Europe, people practiced residential segmentation by ethnic group based on their country of origin, such as England, Scotland, Ireland, Italy, Germany, Poland and Greece. Mutually recognized prejudice or contempt between different communities was widely observed. Furthermore, Jewish people who had been subject to discrimination in Europe were an important immigrant population in America. Many people also moved from Asia to live in the USA. On the other hand, the indigenous people were initially called Indians (derived from Columbus’s misapprehension that America was actually the Indies, or what we would now call Asia), and subsequently Native Americans, but as white settlers drove westward, they were excluded and in due course ended up being “protected” on reservations.

 The American society that was formed from these racially and ethnically diverse and (below) racial and ethnic groupings has been called a “melting pot,” but it has been pointed out that in reality, the different peoples did not melt together as if in a crucible, and that society instead came to resemble a “salad bowl” in which a variety of vegetables are mixed together. Sociologist Emory S. Bogardus argued that it was possible to measure the degree of closeness or estrangement between different racial and ethnic groupings, and proposed a scale of social distance. This scale consists of seven levels representing the way different social groups view each other, from the most receptive attitude of “Would accept them as close relatives by marriage,” through the intermediate attitude of “Would accept them as neighbors on the same street,” to the most exclusionary attitude of “Would eject them from the country.” This scale was used in American sociological research in the 1930s when racial and ethnic discrimination was overt, but today its reasonableness is questioned.

Would different groups with significant social distance (to the extent that they would not “accept [each other] as neighbors on the same street”) really form a ◎◎ district? The segregation model addresses a classic piece of research that attempts to resolve this problem head-on (⇒Reference to segregation model).

Segregation model

 Many large US cities are divided into neighborhoods that are “white” and neighborhoods that are “black,” a feature that is thought to be the result of racial prejudice and discrimination. Given this, what strength of prejudice will generate what level of segregation? The individual who took up the challenge of answering this question head-on was Thomas Schelling, an economist at Harvard University. The framework that he devised is known as the Schelling model of segregation.

  The original model was constituted as follows (simplified for the purposes of explanation). A neighborhood is divided into 64 squares in an 8×8 matrix, in which live 23 “copper” families and 22 “silver” families (the ratio of empty houses is about 30%). For each family, if at least one-third of the families around it are of the same color, the family will continue to reside there (is satisfied), but if it is less than one-third it will move to an empty house nearby. When all the families stop moving, the neighborhood has reached equilibrium. Schelling performed this experiment using a chessboard, 1-cent coins (copper pennies), 10-cent coins (silver dimes), and dice. This experiment is held to be an example of one of the first generation of multi-agent simulations (agent-based simulation).

 The results were surprising. Although for each family, if at least one-third of the families around it are of the same color, the family will continue to reside there (is satisfied), the final result after reaching equilibrium was that families of the same color occupied around two-thirds of the surrounding houses. Let us take a look at the actual Schelling model ( Reference to the original Schelling segregation model). To put it another way, even in the case of individuals who are broad-minded when it comes to different people, the community as a whole is clearly segregated as if the residents are avoiding each other. This is an example of an emergent phenomenon in which the macro aspects cannot be predicted from the micro settings. The same phenomenon can be observed even in larger communities (⇒ Reference to general residential segregation model).

 The situation today is that the composition of the population of the United States is complicated, consisting not only of white and black individuals, but also Hispanics (those originating from Spanish-speaking countries such as Mexico) and those of Asian ethnicity, who are sometimes bundled together, and sometimes classified separately as Japanese, Korean, Chinese, Vietnamese, etc. (Reference to research more closely aligned with the actual situation, for example, Ito & Yamakage 2016 [Copyright Aoyama Gakuin University Society of International Politics, Economics and Communication])).