Residential Segregation

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Introduction
Segregation refers to nothing more than the "disproportionate distribution of population groups across urban sub-units" [1] (p. 217).It can be conceived very abstractly as the unequal distribution of "elements over subunits of a unit" (ibid.)."Elements" can be occupational or income groups, as well as religious groups, age groups, or people of a certain origin."Sub-units" are usually residential areas of the "unit" city.
Issues of residential segregation have been studied on an international scale.They have been subject to longstanding political and theoretical debates and form an inter-and transdisciplinary field of study.Residential segregation includes various dimensions and domains, and a range of methods has been developed for measuring segregation.
The extent of the unequal distribution of the described characteristics can thus be expressed by different statistical measures.The segregation index [2], which measures the distribution of a population group in relation to the total population, is particularly frequently used in research.The index can take values between 0 and 100 and indicates the proportion of a population group that would have to move in order to achieve an equal distribution across the city.A value of 0 stands for a completely even distribution of the examined characteristic, and a value of 100 for complete spatial separation.However, great caution must be exercised when interpreting index values [3] since they depend strongly on the underlying division of the city as a whole into subareas.The larger the subareas, the more likely they are to contain different population groups and the lower the segregation index.In addition, there are different characteristics of urban areas (e.g., building density, physical conditions and building types).For this reason, the comparability of segregation indices between different cities is very limited.
Over the past 100 years, various approaches have been developed to explain segregation.Essentially, six currents can be distinguished.

Socio-Ecological Approaches
Socio-ecological approaches have dominated research on segregation for a long time.Originally, they were developed by the so-called "Chicago School" of urban sociology in the 1920s.In particular, Robert Park, Ernest W. Burgess and Roderick McKenzie [4] advocated the concept of "social ecology", which they used to explain the development of cities in terms originating from zoology and botany.Central to this approach were the concepts of "invasion", "succession" and "dominance".According to the views advocated by the "Chicago School", stronger "species" prevail in the competition for residential locations in a city, each of which moves into the areas that suit their way of life and displaces weaker "species" there.The result is a mosaic of "natural areas" in which city dwellers live together and share norms, lifestyles and behavior patterns.
Based on socio-ecological approaches, "factorial ecology" emerged as a field of investigations and research designs.In a way, this research direction represents the methodological counterpart to the Chicago School and tries to determine segregation by applying complex calculation methods of multivariate statistics (factor-analytical methods).
Typically, such studies [5][6][7] focus on the distribution of population groups across spatial units [3].Often, they use the index of dissimilarity (see above) to measure the intensity of segregation as a function of urban-ecological characteristics.Compiling data on the number and proportion of age groups, ethnic backgrounds, occupation, unemployment, social welfare and other factors on the smallest units covered by public statistics, these studies use linear regression to calculate the concentration of the examined characteristics in different urban subareas.The results are usually very detailed descriptions of segregation with a plethora of data-but less an explanation of the process.

Neoclassical Approaches
Neoclassical approaches to residential segregation were developed in economics.Their explanation for the socio-spatial differentiation of cities is mainly based on an analysis of demand patterns of individual households.The basic assumption of this research is that households aim to optimize the balance between costs and benefits in their choice of residential location [8].The central factors considered in this regard are the location of a site and the associated transport costs.With increasing distance from the center, transportation costs increase, while housing costs decrease.In this model, the choice of a housing location is based on the individual preference curve of a household, i.e., the trade-off between the consumption of living space and quality compared to the transportation costs and in relation to the available income.Alonso assumed that higher-income households prefer residential locations in the less densely populated urban peripheries with lower land prices because these locations allow for optimizing the relationship between land price costs (and thus housing consumption) and transportation costs.Conversely, lower-income households would be dependent on locations close to the city center, as they can only afford less living space and therefore have to live closer together.
Closely related to this branch of research is the so-called "filtering theory" of the housing market [9].It assumes that households move to better housing when incomes increase or the supply of housing expands so that the lowest quality housing units "filter out" of the market.A change in housing supply thus "trickles down" to all submarkets and stimulates residential mobility as new housing options become available.The implicit effect is a change in the social composition of the affected areas.

Behaviorist Approaches
Neoclassical models were met with criticism early, as their strong reliance on economic factors was seen as problematic.Consequently, neoclassical explanations were supplemented by behavioral and life-cycle models in the 1970s and 1980s [10], which provided a more complex picture of household decision making.
The central point of reference for this research direction are household characteristics and their changes over the life cycle.It was assumed that households strive to meet their changing housing needs in close relation to the life cycle (single, household with/without children, senior household) by adapting their living environment to their needs [11][12][13][14].Biographical events were thus regarded as central in determining housing behavior.
Further developments of this theory, however, advanced this perspective and focused more on housing stress, i.e., on the difference between desired and real housing situations [10,15].The basic idea here was that a gap between perceived housing needs and the actual housing situation can lead to "housing stress", which motivates households to change their housing environment.Stress triggers were seen as rooted in the life cycle but also in changes in housing quality or the neighborhood.
Closely related to "stress theories" were contributions which examined the actual decision-making process of households [16].The result of this strand of research was greater incorporation of "context", as it became clear that housing decisions needed to be analyzed as embedded into a "bundle" of trade-offs between living circumstances, financial capacities, cultural preferences and assessments and other factors, as well as in relation to the currently occupied home and the theoretically available alternatives [17].
Overall, behaviorist approaches provided fundamental concepts to understand the housing-related choices of individual households.Although a reference to segregation is not always recognizable in behaviorist studies of residential mobility [18], this approach was nevertheless influential for segregation research, as it enabled an understanding of the causes of residential mobility.However, the focus of this strand of research always rested on the demand side, i.e., on households, their housing needs and their individual choices.

Institutionalist Explanations
Contrasting neo-classical and behaviorist approaches, institutionalist theories developed by British sociologists in the 1960s and 1970s emphasized the importance of housing supply for residential segregation.Central to this approach was a monograph under the title "Race, community and conflict" [19], in which John Rex and Robert Moore used the example of Birmingham to describe how white working-and middle-class households could obtain social housing or owner-occupied homes, while access to both market segments was systematically blocked for immigrants by mortgage institutions and local welfare administrations.As a consequence, immigrants therefore were pushed to private rental housing market, which in British cities was mainly found in the inner city.The explanation for the concentration of people with an immigrant background was thus found in the allocation practices of gatekeepers or "urban managers" [20], who allowed-or denied-certain population groups access to their properties.
Following Max Weber, Rex and Moore used this as a basis for constructing different "housing classes", which they distinguished in relation to their different positions in the housing allocation system.However, transferring the approach to other cities and countries proved difficult.It was also criticized that the category of gatekeepers was descriptively derived from empirical data and appeared to be detached from economic and social conditions and foundational theories about society.Nevertheless, this work allowed for a change in perspective that brought the practices of housing providers more into focus for segregation research.

Political Economy Approaches
A more social theory-based explanation of segregation was offered by Marxist approaches developed since the 1970s.These approaches consider the unequal distribution of housing as a result of the interaction of land rent capture and capital accumulation.Two arguments are central: First, political economy approaches assume that urban housing markets are divided into submarkets, in which landowners develop suitable exploitation models for different demand groups.In this view, suburban "gated communities" and inner-city "ghettos" merely represent different models of land rent capture that result from the actual economic context in which they are positioned [21].Second, processes of uneven development, i.e., the uneven spatial development of capital accumulation, lead to invest-ment cycles, which are expressed in a "see-saw movement" of investment, disinvestment and reinvestment into the built environment.In this perspective, capital is directed into certain housing stocks (and thus valorizes them) and withdrawn from others as an outcome of contradictions inherent in the capitalist mode of production.Segregation is thus, as the title of a foundational paper put it, "a movement by capital, not people" [22].In contrast to other explanatory approaches, political economy explanations thus integrate the study of segregation into a broader social analysis that focuses on the idiosyncrasies of land as a commodity in capitalist economic processes.The integration of individual agency into the macro-theoretical framework of these theories has remained a thorny problem that has repeatedly attracted criticism.

Restructuring Approaches
Finally, a sixth group of explanatory approaches, which has been particularly prominent in the literature about residential segregation in the subsequent three decades, focuses on the broader processes of economic and social restructuring that have caused profound changes in income structures, housing provision and state welfare policies.Starting from the observation of an increasingly unequal income distribution together with increasingly divided housing markets, the first contributions described a "polarization", "dualization" or even "quarterization" of cities [23][24][25][26], expressed in an increasing coexistence of upscale housing, gentrification and abandonment.In the 1990s, these approaches were complemented by new contributions which focused on the influence of welfare state regulations as a crucial intervening variable.While, initially, the focus of this research was on Western European cities [27,28], the perspective was successively broadened, and more geographically diverse sets of cities were analyzed [29][30][31][32].Overall, the research designs associated with restructuring approaches present themselves as very heterogeneous.What the contributions have in common, however, is an interest in a stronger social and geographical contextualization of segregation processes.

Applications
As residential segregation can be regarded as the spatial expression of social inequalities, the theme has found broad interest in research, practice and even everyday language.Folk terms such as "immigrant quarter", "yuppie neighborhood" or "suburb" not only reflect our everyday experience of cities but also describe how diverse patterns of social inequality are reflected in space.It thus hardly comes as a surprise that the description, measurement and explanation of segregation has been a core topic of urban research for more than a century.
Research on segregation is also widely applied by public administrations, housing market analysts, urban planners and other practitioners whose work is influenced by the spatially uneven distribution of population groups.A concentration of social problems, such as unemployment and poverty in individual neighborhoods, affects a wide spectrum of urban planning issues, and, as a consequence, there is a need to monitor such developments on a small-scale basis.Segregation research thus provides important tools for city administrations, not only in the sense of an early warning system but also for selecting areas for funding programs, infrastructural planning and housing policies.
It should be emphasized, however, that the normative and practical stances towards residential segregation vary greatly across times and places.Thus, in the 20th century, practices of "racial steering", i.e., directing homebuyers into neighborhoods based upon race and ethnicity, were widespread in in the U.S. and, for a long time, even supported by public administrations.These policies assumed that people should live in neighborhoods with people of their own ethnicity.The side-effect was, however, discrimination of minority ethnic groups and a downward spiral effect on the neighborhoods in which these were concentrated so that the pattern became self-reinforcing [33].
Closely connected, the self-segregation of middle and upper social groups has significant consequences for segregation but is not often seen as a problem by most planners.This is, to some degree, deficient as segregation can only adequately be understood as a relational process occurring in the whole system of city neighborhoods.The neighborhood choices of more privileged groups thus have significant consequences for the segregation of less privileged groups as well, for example, in the form of gentrification [34,35] and the displacement of low-income groups from the affected neighborhoods.The growth of gated communities [36][37][38] and other forms of enclaves also contributes to social segregation.Luxury and abandonment are therefore interconnected [39].They form two sides of the same coin.
In effect, the segregation of different groups of urban residents according to socioeconomic or ethno-cultural characteristics is therefore neither an exception nor anything new in principle.Rather than describing its patterns as such, it is thus much more important to also ask about the consequences of segregation for individuals and households.In media discourse and planning practice, this is often performed on the basis of so-called neighborhood or context effects.These rest on the idea that "where one lives" has an effect on individual life chances.Consequentially, it is often assumed that a concentration of socially disadvantaged residents has additional disadvantaging effects on people living in the respective neighborhoods.Typical problems mentioned in literature and public debate include increased mortality and health problems, teenage childbearing, low school success rates, delinquency, and even homicide, as well as labor market success in general.The mechanisms leading to these malaises are, however, discussed in a very fragmented way, and different arguments stand side-by-side.Popular assumptions, in this respect, are isolation from mainstream networks and norms, a deviant "culture of poverty", stigmatization effect, a lack of social control, as well as poor quality services and infrastructures-which are all said to have a problematic effect on the life chances of residents in disadvantaged neighborhoods.
While the assumption of neighborhood effects has great significance for urban policy and planning, it has also often become normatively charged.The housing industry, politicians and administration often see segregation (especially of ethnic minorities) as problematic to integration and advocate for "mixed communities".Interestingly, the attention is thereby nearly exclusively on the "negative side", i.e., on the concentration of poor and disadvantaged households.The concentration of better-off persons in gentrified neighborhoods or gated communities receives far less attention.
As such, mixed community policies are based on remarkably thin evidence when subjected to rigorous evaluation [40,41].Moreover, it also found that there are real advantages to living in segregated neighborhoods for particular social groups, e.g., for immigrants who benefit from the concentration of "arrival infrastructures" [42].Mixed neighborhood policies risk destroying these benefits.Research has also shown that simply having resourcerich and resource-poor people living side by side does not necessarily lead to more diverse social networks.Contrasting the expectations of policy-makers, social mix policies have even been blamed as "gentrification by stealth" [43].
Compared to the widespread support for "mixed communities" by politicians and planners, the position of academic research on the relevance of neighborhood effects is much more mixed and heterogeneous [40] (p.160ff.for a short discussion).In urban studies, sociology and human geography, it is commonly accepted that segregation has an additionally disadvantaging effect only under very specific conditions.Only when poverty, social exclusion and disadvantageous housing conditions (e.g., poor housing quality, lack of adequate infrastructure, poor accessibility, high emissions, problematic image, etc.) come together can segregation become a "trap" and have a detrimental effect on disadvantaged groups.Whether a spatial concentration of social groups is "problematic" in this sense cannot be deduced from share values of, for example, people with a migration background alone.It is not so much the composition of residents that matters, but above all, how the different groups interact on the ground, how they recognize each other, whether they feel they are integrated in the essential areas of everyday life, and so on.

Conclusions and Prospects
The brief review of more than a century of research and theory building has made it clear that there is no universally accepted, all-aspects-integrating explanation for residential segregation.Rather, different theories reflect different philosophical traditions, methodological orientations and disciplinary backgrounds [for a recent overview, please see [44].At the same time, they emphasize different aspects of the complex phenomenon of segregation.As a process, segregation is both a result of individual choice (which neoclassical and behaviorist approaches focus on) and of community-building, socialization and stratification processes (which stand in the center of socio-ecological approaches).Access to the housing stock is regulated by gatekeepers (institutionalist approach), and the development of the housing supply follows economic calculations that are part of broader dynamics of capital accumulation (political economy approach).Moreover, housing behavior, class and stratification, and investment approaches are embedded in a context of broader social restructuring (e.g., the transformation from an industrial to a service society), without which they can hardly be adequately explained.In this sense, the explanations for segregation discussed above illuminate different facets of a common theme.Although they can be analytically separated, their interaction is important for a deeper understanding of segregation.
In view of this complexity, there is a growing understanding among scholars that residential segregation is mediated by a broad range of factors, including individual household characteristics and preferences, labor markets, welfare arrangements and housing supply, as well as the city's history, morphology and even topography.All these factors are interrelated and codetermined, and their individual weight can hardly be isolated and extrapolated.In sum, residential segregation thus needs to be understood as a "systemic contextual process embedded in the wider society, its principles of stratification and the role of the state-market [...] nexus in the organization of welfare arrangements, housing, planning and land systems" [31] (p.300).
In contrast to media accounts and assumptions of conventional policy approaches and planning strategies aiming at "mixed communities", segregation is not problematic as such.Arguments supporting "social mix" are often faith-based and built on thin evidence.The term segregation should therefore be applied in a non-normative and precise way and discussing its consequences should only be conducted with considerable caveats.Proportions of population groups in spatial areas can neither successfully guarantee social integration nor do they necessarily lead to social disintegration and individual disadvancement.
Nevertheless, segregation research provides important perspectives through which the spatialization of social inequality can be understood, described and analyzed.It is therefore indispensable both for theorizing about cities as well as for social policy and planning practice.