Editorial on the Special Issue "The Role of Internal Migration as a Driver of Regional Population Change in Europe"

Europe has experienced a growing divergence of trends in population growth and age structure across cities and regions. A key driver of this divergence is internal migration, which also drives disparities in labour markets and economic development. This special issue focuses on the role of internal migration as a driver of regional population change in Europe, and relates current research to the early works on the “laws of migration” by Ernst Georg Ravenstein. The topic of internal migration and regional population change is important and timely, given the ongoing social scientifi c and political debate within Europe about the causes and consequences of regional disparities and the design of appropriate policies to reduce inequalities. The European Union, for example, has the goal of reduction of inequalities across member states and across regions within them. The instruments for achieving this goal are the Cohesion Policy (European Commission 2020a), the Regional Development Fund (European Commission 2020b) and the Social Fund (European Commission 2020c). Previous research on the role of internal migration on past and future population dynamics within the context of regional disparities has largely focussed on population projections at regional and national scales. This includes a project on Demographic and Migratory Flows affecting European Regions and Cities (DEMIFER) (ESPON 2013). One task in DEMIFER was to forecast regional populations and therefore the component fl ows (births, deaths and internal, inter-member state and extra-European migration), with specifi c scenarios showing outcomes under different policies (Rees et al. 2012). Because of time constraints, the scenarios for migration assumed convergence, divergence or stasis in the attractiveness of regions to the three spatial categories of migrants, uninformed by an analysis of trends over space and time. More recent projections of European country populations have adopted either high, medium and low scenarios (Cafaro/Derer 2019) or combinations of high, central and low with differing levels of human capital of migrants (Lutz et al. 2019). However, EUROSTAT have not published regional population projections since 2008. There is therefore a gap in our knowledge of migration trends across Europe which could inform population projections under plausible policy Comparative Population Studies Vol. 44 (2019): 533-544 (Date of release: 08.09.2020)


Introduction
Europe has experienced a growing divergence of trends in population growth and age structure across cities and regions. A key driver of this divergence is internal migration, which also drives disparities in labour markets and economic development. This special issue focuses on the role of internal migration as a driver of regional population change in Europe, and relates current research to the early works on the "laws of migration" by Ernst Georg Ravenstein. The topic of internal migration and regional population change is important and timely, given the ongoing social scientifi c and political debate within Europe about the causes and consequences of regional disparities and the design of appropriate policies to reduce inequalities. The European Union, for example, has the goal of reduction of inequalities across member states and across regions within them. The instruments for achieving this goal are the Cohesion Policy (European Commission 2020a), the Regional Development Fund (European Commission 2020b) and the Social Fund (European Commission 2020c).
Previous research on the role of internal migration on past and future population dynamics within the context of regional disparities has largely focussed on population projections at regional and national scales. This includes a project on Demographic and Migratory Flows affecting European Regions and Cities (DEMI-FER) (ESPON 2013). One task in DEMIFER was to forecast regional populations and therefore the component fl ows (births, deaths and internal, inter-member state and extra-European migration), with specifi c scenarios showing outcomes under different policies (Rees et al. 2012). Because of time constraints, the scenarios for migration assumed convergence, divergence or stasis in the attractiveness of regions to the three spatial categories of migrants, uninformed by an analysis of trends over space and time. More recent projections of European country populations have adopted either high, medium and low scenarios (Cafaro/Derer 2019) or combinations of high, central and low with differing levels of human capital of migrants (Lutz et al. 2019). However, EUROSTAT have not published regional population projections since 2008. There is therefore a gap in our knowledge of migration trends across Europe which could inform population projections under plausible policy assumptions and a lack of an update-to-date, Europe-wide set of projected regional populations for 2020 to use in monitoring infections and mortality rates during the Covid-19 pandemic, across the European Union. This special issue of Comparative Population Studies (CPoS) fi lls a serious gap in our understanding of the migration components of population change, which would also facilitate a more effective response to crises such as the Covid-19 pandemic.
This special issue presents new fi ndings and insights into the role of migration in the development of the populations of cities and regions in Europe. All papers link their fi ndings about contemporary migration fl ows with the "classic" papers by Ernst Georg Ravenstein, who studied internal migration in the British Isles and in a selection of European countries in the nineteenth century. One hundred and thirty years ago, Ravenstein studied the role of migration as a driver of regional population change and published two seminal papers on the Laws of Migration (Ravenstein 1885, 1889). Ravenstein's fi rst paper describes the spatial patterns of internal migration in the British Isles, while the second discusses internal and international migration and population change in Europe and North America.
The remainder of this editorial is structured as follows: Section 2 lists the papers that constitute this special issue and describes their features, so that readers can choose the paper closest to their interests or organize their reading of all papers in a sensible order. Section 3 links the papers to the work of Ravenstein, showing which "laws" are evaluated and how. Section 4 summarises the contribution of the Special Issue to scholarship in internal migration studies and refl ects on what the current Covid-19 pandemic might mean for migration intensity, migration distances, migration directions, migration variation by life course stage and migration impact. In this editorial, when we refer to "migration", we normally mean "internal migration" or place to place changes of usual residence within the borders of a nation state. Occasionally, reference is made to international migration between countries. Throughout the text, we use author names in italics to make reference to the special issue papers.

Papers in the special issue
The eight papers that are included in this special issue either focus on multiple countries in Europe and beyond to adopt a cross-nationally comparative perspective, or on a specifi c country (England and Wales, Germany, Spain, Italy and Russia). Table 1 sets out details of the papers in the special issue. All papers belong to Volume 44 (2019), but because CPoS is an online open-access journal that publishes articles on a continuous rolling basis, some of the papers were published in 2020. Table 2 summarises the content of each paper in terms of spatial patterns, contribution to population change and the characteristics of the data sets employed. The fi rst column of the table lists the family names of the authors. The second to fourth columns give descriptions of the data used in each paper, identifying the country or countries studied, the spatial zones used, and the time periods for which data were available. The fi fth to seventh columns describe the key themes of each paper, in Tab terms of migration patterns, contributions to population change and whether different population groups were considered in the analysis.
The migration data used in the special issue papers vary in spatial and temporal scale due to cross-country variations in the defi nition and measurement of migration fl ows (see Table 2). Rees and Lomax review measurement and analysis approaches adopted by Ravenstein and contemporary researchers. Illustrations of the main arguments are drawn mainly from the United Kingdom and decades from 1990-91 onwards. Maps of net internal migration at local government district scale are used to illustrate changes in spatial variation over a recent decade. Rowe et al. analyse migration patterns in 28 European Union member states (only 27 since 31 January 2020), using the smallest spatial unit available in each country. Day uses census microdata for England and Wales. He employs scales ranging from parishes

Themes of the special issue papers
We provide a brief summary of the rich seams of analysis and fi ndings presented in the eight papers. We do this by commenting on the relationships between paper fi ndings and Ravenstein's "laws of migration" (empirical generalisations), using the interpretation of these in Grigg (1977).

How does migration analysis in recent decades differ from that of Ravenstein?
Rees and Lomax review how migration is defi ned, how it is measured, and through what instruments the data are collected (censuses, surveys, registers, telecommunication and internet sources). The paper identifi es the type of migration measure used by Ravenstein: tables of counties of birth and residence at a census, collectively termed "lifetime migration". Because these data do not pin-point when the migrants moved, they have been rarely used in analysis subsequently. The paper discusses methods developed recently for inferring between country migration for fi xed time periods from collections of census and survey tables of the population classifi ed by country of birth and country of residence, triggered by key papers by Abel (2013) and Abel and Sander (2014).

Migration and distance
Ravenstein: The majority of migrants move only over short distances (Grigg 1977 The development of gravity models of migration embodying distance decay functions post-dates Ravenstein's papers. However, it is diffi cult to compare the decay parameters between countries because they are highly dependent on how many regions are used and what their size is. Rees

Migration and settlement hierarchies
Ravenstein: Migration proceeds step by step (Grigg 1977, Law 2) Ravenstein's generalisation that migration proceeds step by step has been interpreted and investigated in two different ways. The fi rst interpretation is that it represents the hypothesis that migrants, over the course of their lifetime, make a chain of migrations through a sequence of successively larger settlements. The second interpretation is that the sequence of fl ows occurs at an aggregate, not an individual scale. For example, there might be a net fl ow from county A to county B and from county B to county C. County A might be highly rural, county B might consist of mixed urban and rural settlements and county C would be highly urban. But different people would be involved in each step of the fl ow sequence. Ravenstein coined his generalisation by mapping selective chains of migration fl ows. Day provides an insightful review of understandings of the step by step migration hypothesis and demonstrates, using census microdata and parish registers and other evidence, that most migrants in 19 th century England and Wales took only one step from starting home to a bigger place. Champion uses aggregate migration fl ow data from patient registers to demonstrate that internal migration fl ows to and from the major metropolitan area of London constitute an outward, step by step downward cascade, providing evidence that migration fl ows not only proceed up the settlement hierarchy but can also point downwards.

Migration, Urbanisation and Economic Development
We group the following Ravenstein generalisations together, because of their close relationships.
• Migrants proceeding long distances generally go to the great centres of commerce and industry (Grigg 1977, Law 2) • Towns grow more by migration than natural increase (Grigg 1977, Law 8).
• Migration increases as industries develop and the means of transport improves (Grigg 1977, Law 9).
• The major direction of migration is from the rural areas to the towns (Grigg 1977, Law 10).
• The main causes of migration are economic (Grigg 1977, Law 11).
The discussion about the aggregate form of the step by step hypothesis is closely linked to the debate about the extent to which migration leads to urban concentration/urbanisation (driven in part by net migration gains in urban centres) or de-concentration/counter-urbanisation (driven in part by net migration losses in urban centres). Urbanisation has been the dominant global trend through most of the 19 th and 20 th centuries, but de-concentration was observed in some countries in Western Europe from the 1970s onwards. Because it is diffi cult to defi ne urban, rural and intermediate regions in a comparable way across countries, researchers have used an indirect method to track the phenomenon: measuring net migration rates for territorial units and tabulating or regressing them against the units' population density (Rees et al. 2017).
Stawarz and Sander examine the degree to which internal migration in Germany has been "to the great centres of commerce and industry" (Ravenstein 1885: 199) or away from them, using a regression method. They show that in recent decades, the patterns of internal migration have swung back and forth between urbanising and counter-urban tendencies, causing population gains in either the inner cities or their hinterlands. These swings are related to changes in urban and rural labour markets, the rise in the higher education industry in cities and the role that rising housing prices have on preferences for urban, suburban or rural residence among families and older adults. The impact of internal migration on population change varies over time and country: that is, it is context-dependent.
Rowe et al. produce a summary graphic of changes in net internal migration rate and population density relationships for four European countries over the 1995 to 2010 period. Each country follows its own path. Their table for 10 countries in the 1980s-1990s and 2000s-2010s shows the same mixed picture as Stawarz and Sander.

Migration differences across population groups
Ravenstein: Most migrants are adults; families rarely migrated (Grigg 1977, Law 7). Ravenstein: The natives of towns are less migratory than those of rural districts (Grigg 1977, Law 5). Ravenstein: Females are more migratory than males within the kingdom of their birth, but males move more frequently abroad (Grigg 1977, Law 6).
Ravenstein did not have access to census migrant tables disaggregated by migrant characteristics. This was the era of counting of the census returns by hand. It was not until 1884 that Herman Hollerith patented his punched card tabulator (Wikipedia 2020). This made production of multiple census tables possible before the 1960s, when mainframe computers took over, to be replaced by mini and personal computers and servers from the 1990s. Analysis of population behaviour by life course variables such as sex, age, birth cohort and status within the household are now the foundation of much demographic and health analysis. Internal and international migration are routinely classifi ed by these demographic attributes. The papers in the special issue naturally refl ect their importance. For example, Rees and Lomax show for Great Britain in 1990-91 that the strength of the net internal migration-density relationship varied by life course stage, with de-concentration the dominant process in the family and working ages, with urbanward migration important only in the late adolescent and young adult ages. Karachurina and Mkrtchyan demonstrate that, for Russia, inter-regional migration fl ows are dominated by young adults migrating to regional centres and large metropolises, particularly Moscow, for higher education and training. Gil-Alonso and Thiers-Quintana and Benassi et al. fi nd, for Spain and Italy respectively, that foreigners are much more active in internal migration to and between metropolitan areas than are natives.

Effectiveness of migrant fl ows
Ravenstein: Every migratory current has a counter current (Grigg 1977, Law 7) Rowe et al. use the Migration Effectiveness Index (MEI), which is the scaled absolute value of a given directional migration fl ow minus its counter-fl ow divided by the sum of fl ow and counter-fl ow. High values of MEI indicate a substantial imbalance in fl ow volumes and that internal migration is altering the population distribution. The MEI is a building block along with the Crude Migration Intensity (CMI) of an overall Index of Net Migration Impact (INMI). The authors show that the same amount of impact can be generated by a high CMI and low MEI as a low CMI and high MEI. For example, both Russia and Finland have almost the same INMI values. Russia's INMI derives from a low CMI and a very high MEI, while Finland's derives from a high CMI and low MEI. The analysis thus confi rms Ravenstein's observation that every fl ow has a counter-fl ow, although the counter-fl ow may be much smaller in size, leading to population redistribution.

The relationship of internal and international migration
Some papers focus on internal migration, such as Rowe et al. and Karachurina and Mkrtchyan. Other papers include a consideration of international migration into and out of the country being studied, such as Gil-Alonso and Thiers-Quintana and Stawarz and Sander. In particular, the role of people with foreign origins in internal migration is analysed by Benassi et al. for Italy and Gil-Alonso and Thiers-Quintana for Spain. Rees and Lomax review the fi elds of internal and international migration in terms of concepts, measurement techniques and analysis methods, comparing current practice with methods available to Ravenstein. They explain the methods developed to estimate international migration between countries in a sequence of papers starting with Abel (2013). The suggestion is made that lifetime internal migration data, available in many countries but not much used because the timing of the migration captured is uncertain, could be used to make estimates of periodspecifi c migration trends.

Discussion
We hope that the outline of the contents of this Special Issue demonstrates the usefulness of looking back at a classic study to make sense of a complex phenomenon such as internal migration. Of course, we have only described a small part of the richness of analysis to be found in the eight papers. How might this knowledge be used? At the start of this editorial, we pointed to the need to develop new projections of regional populations in Europe, using a multi-country and multi-region framework. Such projections were implemented up to 2008 by Eurostat but have since been abandoned. If the opportunity arose to construct such projections, the special issue papers would suggest the projection model needed to take into account the shifts over time in the spatial structure of internal migration fl ows, and to be made operational for a hierarchy of cities, their immediate surrounds and rural areas.
We have prepared this overview of the special issue in spring 2020, a time of severe crisis due to the Covid-19 pandemic affecting all countries of Europe. Human mobility plays a key role in this pandemic because infectious disease spreads through human contact. Based on current knowledge, Covid-19 originated in the city of Wuhan in China in late 2019 (Ma 2020;Davdison 2020), and spread to other countries, particularly those in Europe, in early 2020. Governments throughout Europe took action to reduce social contact. As a result, human mobility was rapidly reduced, spanning from daily mobility patterns (e.g. travel to work), to commuting fl ows and international (labour) migration. India, for example, took the step, with only a few hours notice, to "lock down" the country's 1.3 billion population and precipitated a reverse internal migration from the cities to the countryside in chaotic travel conditions (The Economist 2020a).
The Covid-19 pandemic has, in effect, radically altered patterns and intensities of internal and international migration. However, we cannot yet quantify the ways in which Covid-19 has changed migration due to lags in data publication. Neither we as the guest editors nor the authors of the special issue papers envisaged the virtual disappearance in 2020 of the fl ows under study. However, we trust that all forms of human mobility and migration will eventually resume, if only in subdued form to begin with, and that the fi ndings of this suite of research papers will provide some guide to the future study of migration and its role in regional population change.