Unraveling the (Uneven) Linkage? A Reﬂection on Population Aging and Suburbanization in a Mediterranean Perspective

: A complex interplay between socioeconomic transformations and demographic dynamics has characterized the long-term development of European countries. As a characteristic example of such linkage, the present study focuses on the spatial relationship between metropolitan growth and population age structure. Preferences for urban and suburban locations reﬂect complex socioeconomic phenomena such as sprawl, class segregation, gentriﬁcation and ﬁltering. However, the spatial linkage between sprawl and demographic transitions was relatively poorly analyzed, and should be more extensively investigated in relation with population dynamics and socioeconomic structures at local scale. By reviewing pertinent literature, this study outlines how space exerts a non-neutral impact on population age structures in Europe, shaping housing needs and inﬂuencing settlement patterns and processes of urban transformation. While suburban locations have concentrated younger families and larger households in Northern and Western Europe, the socio-demographic composition of new settlements is increasingly dominated by older inhabitants in the Mediterranean region. Results of this work suggest how discontinuous urban expansion was speciﬁcally associated with an elder, wealthy population with high standard of living and a preference for speciﬁc housing locations such as detached villas with gardens and swimming pools.


Introduction
A complex interplay between socioeconomic alterations and the underlying demographic trends has characterized the long-term development path of Europe, especially in recent decades [1][2][3][4]. Demographic transitions are considered a sign of "a pathway to change" typical of the contemporary societies and economies [5][6][7]. The first demographic transition resulted in decreasing birth and death rates from traditionally high levels typical of a pre-transitional, stable population state [8][9][10]. The end of this transition in Europe was reflected in population concentration in urban areas between the 1960s and the 1970s [11][12][13]. The subsequent demographic change (hereafter called the 'second demographic transition') was characterized by important social transformations and more latent processes of demographic redistribution across regions [14], triggering a greater heterogeneity in population dynamics, time of childbearing, households' size, individual choices concerning marriage or cohabitation, and aging [15].
While the first demographic transition has reflected intense socioeconomic change along urban-rural gradients [16], impacts of the second demographic transition were more evident when While the first demographic transition has reflected intense socioeconomic change along urbanrural gradients [16], impacts of the second demographic transition were more evident when comparing population dynamics in urban and suburban locations [17]. In these regards, a completely new spatial structure of population and settlements has progressively been established in Europe, replacing the traditional divide in urban and rural areas with a more subtle configuration opposing suburban locations to both central cities and rural districts [18]. Following these transitions, specific processes of family formation such as marriage or cohabitation were particularly variable over space during the second demographic transition [19][20][21], with less polarized gender roles [22][23][24]. At the same time, demographic dynamics resulted that were more sensitive to economic shocks [25][26][27], reflecting intense and non-linear relationships between socio-demographic change and short-term economic downturns [28,29].
The present study focuses on the relationship between urban growth and population age structure as a result of locally differentiated demographic transitions characteristic of long-term development processes in Europe. Taken as an example of the complex interplay between socioeconomic transformations and demographic dynamics, preferences for urban and suburban locations reflect socioeconomic phenomena such as sprawl, gentrification, class segregation, and filtering. Assuming space as a non-neutral factor impacting population age structures, shaping housing needs and influencing settlement patterns and processes of socioeconomic transformations in metropolitan regions, we reviewed pertinent literature to demonstrate that the spatial linkage between sprawl and demographic transitions was relatively poorly analyzed in Mediterranean Europe. A refined understanding of such dynamics requires an extensive investigation of spatially varying population trends and socioeconomic contexts ( Figure 1). While younger families and larger households dominated the socioeconomic profile of suburban locations, new settlement types-especially discontinuous urban fabric-were frequently dominated by older households in Mediterranean countries. More specifically, urban sprawl in Southern Europe was increasingly associated with an elder, wealthy population with high standard of living and a preference for housing segments such as detached villas with private gardens and swimming pools. Since different population segments by age may have distinctive housing needs, an in-depth analysis of preferences for urban or suburban locations is key to understanding latent relationships between settlement patterns and demographic transitions [30,31]. To answer this deserving research issue, the present work provides a review of spatial settlement preferences of young and older households, outlining the specificity of Mediterranean socio-demographic settings with respect to other countries in both Western, Northern and Eastern Europe.

Spatial Preferences of Young
Age is a key factor of individual's residential preferences [32][33][34]. Housing types are assumed to reflect distinctive residential preferences of specific age groups in Europe [35]. Age structure, household size, density of unoccupied houses, and population concentration were demonstrated to differently characterize discontinuous settlements during demographic transitions in Southern Europe [36]. The most recent socioeconomic transitions were associated with a progressive shift from compact cities to polycentric (and spatially balanced) urban settlements [37]. Under stable (or slightly negative) natural population balance (births minus deaths), internal and international migration represent the engine of demographic changes [38][39][40][41]. Internal migration was a traditional process shaping urban-rural divides in Mediterranean countries for decades; working-age international migration fuelled urban growth especially in the last two to three decades [42][43][44]. Immigration from Eastern Europe and non-European developing countries have been particularly strong in highly segregated labor markets up to the late 2000s [45]. Young migrants were particularly incline to settle in core cities, being predominant in low-quality, central neighborhoods [46][47][48] offering job opportunities, better public transport, cheaper housing, and networking [49][50][51].
Age shapes population mobility since younger households endorse sprawl by demanding less busy neighborhoods in quiet and reasonable locations [52], escaping from urban lifestyles typical of modern societies [53]. Young couples with children were particularly demonstrated to establish preferentially in residential rural settlements with green areas [54] because (i) housing prices are usually lower than in strictly urban areas and (ii) suburban settlements can fulfill a specific demand for bigger dwellings that are unaffordable in central cities [55][56][57][58]. Housing preferences of young families in suburban Europe have been investigated since the 1970s, when a large number of people chose to live in large apartments or in detached or semi-detached houses, because of the appropriateness of suburbs for larger households with children [59]. In Mediterranean Europe, however, suburbanization was also associated with additional factors such as proximity to the sea coast and settlement informality [60][61][62][63]. For instance, sprawl in Barcelona was defined as a sort of 'lock living' housing model, leading to landscapes more similar to the United States suburban archetype than to Northern and Western Europe dispersed settlements [64][65][66].
Suburban landscapes in Mediterranean areas are usually constituted of detached houses with gardens and swimming pools, revealing an intrinsic relationship between pool density and household wealth at the local scale [67][68][69][70]. The socio-demographic profile of suburban settlements was dominated for a long time by households typically composed of young couples with children. By contrast, one-component households (e.g., foreign migrants and native older people, young singles, or divorced adults who live alone) showed a preference to live in urban locations, because of higher accessibility to public services [71]. However, immigration from developing countries fueled an intense spatial redistribution of population from compact cities to suburban districts [72]. In countries such as Italy and Spain, the most significant factors driving spatial mobility were (i) the increasing population pressure in compact cities as a result of the "baby boom" of the 1970s, (ii) rising house prices in the 1980s, as well as (iii) land availability constraints and a growing demand for more living space coupled with increasing household income in the 1990s [73,74]. For instance, compact residential urban settlements in Catalonia rose from 379 to 409 km 2 between 1987 and 2002, growing by 7.9% (European Environment Agency, 2006). By contrast, low-density urban land area increased from 271 to 581 km 2 , rising by 114% in the same time period [75]. In Spain, low-density settlements were demonstrated to reflect cultural changes, socio-demographic transformations and a generalized growth of disposable income for households [76].
Socioeconomic variables such as income or citizenship also have a greater weight on car ownership. Assuming that education level is associated with job employment and working location [23], high incomes account for a greater use of car, whereas private mobility decreases for workers with lower education levels and foreigners. Car ownership in families residing in high-density settlements is much lower than in families settling the new dispersed suburbs [77]. A high number of graduates with jobs settling in central cities results in a lower use of private transport [78]. In traditional compact cities, journeys are shorter and car use is relatively low [79], with higher densities and land-use mix enabling residents to perform more activities in a single journey. By reducing the total number of trips by 20%, compact settlements facilitate foot or bicycle mobility and the provision of public transport [80].
While young people who want to create a family prefer suburban locations, younger people-students, early workers-demand urban locations (Christie et al., 2017; Tucker-Raymond and Rosario, 2017), being mostly flat sharers [81][82][83][84][85]. This phenomenon has been more recently observed in Southern Europe than in Western and Northern Europe, occurring primarily in 'university' cities [86]. At the same time, one-person households and cohabiting couples below 35-40 years also play a role in re-urbanization dynamics [30]. Re-urbanization is a comparatively recent phenomenon in Southern Europe [31], consolidating residential functions in central cities [87]. Re-urbanizing Mediterranean cities, however, was demonstrated only in few cases, since urbanization stimuli still coexist and interconnect largely with other socio-demographic processes including settlement sprawl, class segregation, gentrification, and filtering [88].
Empirical results of earlier studies suggest that the impact of age on residential dynamics basically dealt with life course, household size, urban cycle, and economic downturns [49]. Young adults' choices to live in inner cities (or outer suburban areas) depend on household composition and social status [89]. Furthermore, the increasing diversity of lifestyles and household types suggests that age could be less significant in defining residential location in the coming future [90]. Conversely, the rising segregation of age groups in education, and the division of labor and consumption practices, outline a rising importance of age in defining residential contexts [91][92][93].

Spatial Preferences of Elders
Europe faced a huge expansion of residential settlements in urban areas [42,53,63,[94][95][96]. The Mediterranean region has experienced continuous urbanization occurring especially along the coastline and in internal lowland [97][98][99][100]. In flat areas, sprawl was particularly evident in the last decades, because of the uneven expansion of urban networks fueled by service-oriented economies and tourism [101][102][103]. Together with economic growth and tourism, one of the driving forces was international retirement migration, a recent process reflecting the increasing residential movements of retired people from different parts of the world [104]. Different explanations for the rapid increase of international retirement migration, for example, from Northern to Southern Europe or from Canada/United States to selected Caribbean islands or Florida, were increasingly studied [105]. International retirement migration embraces an aging population, delay of active old age, improved but differentiated disposable income, varying retirement patterns, and enlarged familiarity with 'global' working and vacation patterns [106].
Older adults are less likely to influence their residential environment, as they tend to travel outside their own neighborhoods less often than younger adults do [107][108][109]. However, retirees embody one of the current kinds of population that change their place of residence more frequently than in the past, seeking for a better life quality [110]. Earlier studies investigated the motivations underlying the choice of older people to move abroad after retirement, considering together consolidated lifestyles and social networks [111]. Retirement migration reflects a kind of residential migration, which happens because of an increased demand for 'self-fulfillment' or the 'good life' [112]. North-South retirement migration within Europe can be related back to the 1992 Maastricht Treaty [113][114][115], allowing free movement of European citizens within the continent and giving them the chance to export rights to healthcare and other social security schemes [116]. As a result of European integration, retirees settled preferentially in coastal districts all over the Northern Mediterranean basin, from Portugal to Greece [117]. Four types of older-age migrants were identified in Europe: (i) those who had gone from being a tourist to being resident, whose major explanation for emigrating was the enhancement of life quality owing to, for example, agreeable climatic conditions and lower cost of living [118]; (ii) those who both worked and retired in the destination country [119]; (iii) people retired in their country of origin and regrouped with their children [120]; and (iv) those who immigrated to Southern countries to reside in a dwelling for old people, since it is cheaper than in (origin) countries [121].
The concept of 'international retirement migrants refers to Northern European retirees who decide to reside (seasonally or permanently) in Southern Europe, counting both those who have migrated previously and subsequently their retirement [111]. They are also called "residential tourists" or "lifestyle movers" [122]. Today, Italy, Portugal, and Spain are the preferred countries by people in retirement age (or close to it) from Northern and Central European countries [105,123,124]. These migrants usually display a strong demand for rural lifestyle or a seasonal escape from contemporary life [125]. Residential mobility has rapidly grown since the 1980s because of globalization and background socio-political factors [126,127]. By attracting a substantial flow of residential mobility from Northern countries, Mediterranean regions still remain the most suitable places for retirement migrants in Europe because of climatic conditions and lower costs of living [128][129][130]. For instance, an increasing number of retired migrants from Great Britain decided to migrate to Southern Europe [125]. There has been a long tradition of United Kingdom natives residing in Spain since the 1950s, with a boom in the 1980s; this generation is now ageing. It is estimated that 40% of UK nationals living in Spain are over 50 years of age, and that a large percentage have been living there for decades [98]. These migrants were attracted to Spain thanks to the combination of a high quality of life, warm and sunny climate, and low housing costs-although this has changed to some degree with housing price inflation and, more recently, with a change in the exchange rate devaluing the Pound against the Euro [131].
International retirement migration reflects social consequences that have been underexplored up to now [132][133][134]. This lack of knowledge conflicts with the economic influence this population segment exerts both in their countries of origin and settlement. However, migration studies based on older people have been exclusively focused on wealthy Northern Europeans moving to the sunniest Mediterranean coasts [135]. In the last decades, earlier studies explored social, demographic, and economic consequences of retirement migration, giving only limited attention to negative environmental externalities and indirect impacts on landscapes [136][137][138]. In this regard, it was demonstrated how international retirement migration fueled an increased demand for new residential areas close to natural amenities [97,126,139]. Local and foreign people settled in smaller land plots fueled residential development along the sea coasts [140][141][142], leading to mixed housing typologies and a substantial increase of foreign residential tourists, as observed in Southern Spain with retired British people [112,143,144]. In these regards, some of the largest and most important districts receiving people in retirement age (or close to it) from other countries were located in Spain [99]. In Alicante, an intense real estate bubble in the late 2000s and the continuous arrival of European foreigners altered the urban outline since most development took place on low-priced land in the less-developed provincial areas, while the already urbanized Northern (and richer) areas expanded moderately [145].
Concerning income levels, most retirees worked as skill employees (60%) or as self-employed business owners (25%) before retiring, and 65% had a net month income higher than €1500. Among European retirees, many of them chose to buy their second house in suburban areas. There are different forms of migration at both the international and domestic scale with regard to the second house, arising from tourism and migration processes [146]. Lifestyle choice can exert a significant influence on residential relocation behaviors, particularly at specific stages in the household or individual life-cycles [147][148][149]. Migration to rural areas or peri-urban neighborhoods was related to an intrinsic demand to satisfy lifestyle choices associated to leisure values [150][151][152]. The home situation plays a major role in aging, with older people having a familiar and safe location in which to follow their activities [153].
Second homes contribute to maintain a more familiar environment giving the best opportunity for relaxation and improving the perception of a 'residential idyll'. The desirability of second-home living should be measured within the background of the destination image, assuming that countryside offers a good quality of life. For instance, most second homes in the Alicante region comprised multiple options for activities (e.g., cycling, walking, and golf) [126]. Such patterns have significant implications for the existing farms, since land prices may rise in response to accelerated demand from 'lifestyles' migrating from urban centers and/or non-permanent second homeowners [154]. These processes can be regarded as a sort of rural gentrification leading to fragmented land ownership, increasing human impact and greater disturbance of local ecosystems [155]. In such contexts, native population and migrants (with temporary or permanent second homes) may occupy a 'shared but separate' geographic space with considerable consequences for resilience of local communities, counting environmental and political issues, and long-term sustainability [156][157][158].

Concluding Remarks
This commentary reviews housing preferences of population by age group in Mediterranean Europe. Earlier studies indicate how urban sprawl depends mainly on young people with greater propensity to marriage and childbearing. Younger people prefer strictly urban contexts [159][160][161]. Elders tend to live in consolidated settlements in both central cities and peripheral neighborhoods [131,162,163]. In recent decades, suburbs have become a preferred location for two population segments: (i) young people, who usually move to these locations demanding houses with a garden, and (ii) retired people, choosing these contexts for quietness and mild climate [116]. In fact, retired people preferentially choose the coastal districts of Mediterranean Europe to spend their free time seasonally or permanently. Spain and Portugal are the most coveted contexts, as described in the literature [125]. Generally speaking, rural areas concentrate older residents, because of locally-specific trends in urbanization and re-urbanization [164]. At the same time, the relationship between population age structure and place is not as simple as the rural-urban distinction; for instance, a substantial difference in the age structure of rural population was observed across European regions [73,93,118].
Residential mobility and relocation in quiet, often tourism-related areas with an agreeable climate for the elderly is a recent but problematic phenomenon. Since the 1980s, regional development and wealth redistribution in most European countries have improved the disposable income for vacation purposes [106]. Such welfare added to the growing internationalization of mass tourism, shaping labor migration [156]. Furthermore, the spread of both arrangements of pension schemes (e.g., state, occupational, and personal) and capital accumulation through the property market (usually house ownership or inherited property) has improved prosperity and income availability at the retirement stage of the life course, enabling different types of tourism-related migration [165][166][167]. While considering a wealth of pertinent literature, this study acknowledges the intrinsic limitations of this type of bibliographic analysis that depends on subjective selection of earlier studies. Future research is increasingly required to focus on population aging and sustainable development, linking retirement migration, social vulnerability of the elderly, the decline of suburbs, and distinctive cultural/socio-demographic changes in urban and suburban areas [168].
Since demographic transitions have traditionally paralleled urban dispersion trends [167], the young families of the 1980s, originally settling in low-density contexts in larger houses and having childbearing prospects, are now getting older and restrained in still dispersed settlements where it is essential to use a car to reach services or activities [131]. Such outcomes are rapidly shaping the socioeconomic composition of Mediterranean suburbs, making them progressively more similar to the traditional United States suburbs. While suburbs have been considered as the American 'idyll', having younger families and larger households at the base of the United States dream [169], current population living in such areas is aging [5]. Restricted mobility, especially where substitute transportation options are less available or unreachable [165], is just one of the problems affecting elders in contemporary suburbs. A greater consideration of the multifaceted implications of suburban population aging in Mediterranean areas forms the appropriate background to design strategies enforcing sustainability and resilience of local communities. In addition to transportation, lack of public service infrastructures is a particularly important issue creating insurmountable problems to older inhabitants [170]. Among others, referring to the gerontological literature on 'aging in place' would provide a useful perspective informing a truly sustainable spatial planning oriented toward the specific demands of a progressively older population [168]. Identifying additional issues posing challenges for spatial planning of getting-older settlements is an urgent need for European-and especially Mediterranean-social scientists interested in socioeconomic and environmental dynamics characteristic of peri-urban communities.

Conflicts of Interest:
The authors declare no conflict of interest.