If people tend to live, work, eat or go to school together with other members of their group – race, gender etc. – then we shouldn’t automatically assume that this is caused by discrimination, forced separation, restrictions on movement or choice of residence, or other kinds of human rights violations. It can be their free choice. However, if it’s not, then we usually call it segregation and we believe it’s a moral wrong that should be corrected. People have a right to live where they want, go to school where they want, and move freely about (with some restrictions necessary to protect the property rights and the freedom of association of others). If they are prohibited from doing so, either by law (e.g. Jim Crow) or by social pressure (e.g. discrimination by landlords or employers), then government policy and legislation should step in in order to better protect people’s rights. Forced desegregation is then an option, and this can take various forms, such as anti-discrimination legislation in employment and rent, forced integration of schools, busing, zoning laws, subsidized housing etc.
There’s also some room for intervention when segregation is not the result of conscious, unconscious, legal or social discrimination. For example, poor people tend to be segregated in poor districts, not because other people make it impossible for them to live elsewhere but because their poverty condemns them to certain residential areas. The same is true for schooling. In order to avoid poverty traps or membership poverty, it’s better to do something about that as well.
In all such cases, the solution should not necessarily be found in physical desegregation, i.e. forcibly moving people about. Perhaps the underlying causes of segregation, rather than segregation itself, should be tackled. For example, rather than moving poor children to better schools or poor families to better, subsidized housing, perhaps we should focus on their poverty directly.
However, before deciding what to do about segregation, we have to know its extent. Is it a big problem, or a minor one? How does it evolve? Is it getting better? How segregated are residential areas, schools, workplaces etc.? And to what extent is this segregation involuntary? The latter question is a hard one, but the others can be answered. There are several methods for measuring different kinds of segregation. The most popular measure of residential segregation is undoubtedly the so-called index of dissimilarity. If you have a city, for example, that is divide into N districts (or sections, census tracts or whatever), the dissimilarity index measures the percentage of a group’s population that would have to change districts for each district to have the same percentage of that group as the whole city. Formally:
Where bi is the number of blacks in district i, B is the number of blacks in the city as a whole, wi is the number of whites in district i, and W is the number of whites in the city as a whole. This formula gives you a number between 0 and 1, 0 for no segregation and 1 for complete segregation (similar to the Gini coefficient).
The dissimilarity index is not perfect, mainly because it depends on the sometimes arbitrary way in which cities are divided into districts or sections. Which means that modifying city partitions can influence levels of “segregation”, which is not something we want. Take this extreme example:
The image shows the same city twice, with two different partitions, A and B situation. No one has moved residency between situations A and B, but the district boundaries have been altered radically. In situation A with the districts drawn vertically, there is no segregation (dissimilarity index of 0). But in situation B, with the districts drawn horizontally, there is complete segregation (index = 1), although no one has physically moved. That’s why other, complementary measures are probably necessary for correct information about levels of segregation. Some of those measures are proposed here and here.