Selective migration, health and deprivation: a longitudinal analysis

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Abstract

Population migration is a major determinant of an area's age-sex structure and socio-economic characteristics. The suggestion that migration can contribute to an increase or decrease in place-specific rates of illness is not new. However, differences in health status between small geographical locations that may be affected by the inter-relationships between health, area-based deprivation and migration are under-researched. Using the Office for National Statistics (ONS) England and Wales Longitudinal Study (LS) 1971–1991, this research tracks individuals to identify any systematic sorting of people that has contributed to the area-level relationships between health (limiting long-term illness and mortality) and deprivation (Carstairs quintiles). The results demonstrate that among the young, migrants are generally healthier than non-migrants. Migrants who move from more to less deprived locations are healthier than migrants who move from less to more deprived locations. Within less deprived areas migrants are healthier than non-migrants but within deprived areas migrants are less healthy than non-migrants. Over the 20 year period, the largest absolute flow is by relatively healthy migrants moving away from more deprived areas towards less deprived areas. The effect is to raise ill-health and mortality rates in the origins and lower them in the destinations. This is reinforced by a significant group of people in poor health who move from less to more deprived locations. In contrast, a small group of unhealthy people moved away from more deprived into less deprived areas. These countercurrents of less healthy people have a slight ameliorating effect on the health–deprivation relationship. Whilst health–deprivation relationships are more marked for migrants there are also health (dis-) benefits for non-migrants if their location becomes relatively more or less deprived over time.

Overall we found that between 1971 and 1991, inequalities in health increased between the least and most deprived areas, compared with the health–deprivation relationship which would have existed if peoples’ locations and deprivation patterns had stayed geographically constant. Migration, rather than changes in the deprivation of the area that non-migrants live in, accounts for the large majority of change.

Introduction

Together with fertility and mortality, migration is a major determinant of an area's population structure. In many parts of the UK, at both regional and small area levels, change in population composition due to internal and international migration is greater than the differences resulting from natural change (Champion, Fotheringham, Rees, Boyle, & Stillwell, 1998; Scott & Kilbey, 1999). Moreover, since migrants also differ from each other and from non-migrants, migration events will have different effects on the socio-economic and demographic characteristics of the populations in both the origins and destinations. The suggestion that population migration can contribute to an increase or decrease in place-specific rates of illness is not new (Farr, 1864; Welton, 1872) but, as Verheij, Dike van de Mheen, de Bakker, Groenewegen, and Mackenbach (1998) point out, there is relatively little empirical evidence with respect to the effects of selective migration on geographical variations in health outcomes. In particular, we are interested here in the role that migration plays in influencing the relationship between health and area-based deprivation.

Deprivation is usually taken to be a state of disadvantage relative to the local community or the wider society or nation to which an individual, family or group belongs (Townsend, 1987). People can be deprived of adequate education, housing of good quality, rewarding employment, sufficient income, good health and opportunities for enjoyment. However, a frequent approach has been to calculate an area's deprivation using census-based indicator variables combined into a single deprivation score for each area. Commonly used UK deprivation indexes include the Carstairs Index (Morris & Carstairs, 1991), Townsend Index (Townsend, Phillimore, & Beattie, 1988) and Jarman Underprivileged Area (UPA) Index (Jarman, 1983)1 and in the analysis below we use the Carstairs Index.

After the Black Report (Black, Smith, & Townsend, 1982) revived interest in place effects on health inequalities, numerous research projects have demonstrated that area-based measures of deprivation are related to mortality (Drever & Whitehead, 1995; Eames, Ben-Shlomo, & Marmot, 1993; McLoone & Boddy, 1994; Senior, Williams, & Higgs, 2000; Sloggett & Joshi, 1994), life expectancy (Raleigh & Kiri, 1997), morbidity (Boyle, Duke-Williams, & Gatrell, 1999) and specific health outcomes (Crombie, Kenicer, Smith, & Tunstall-Pedoe, 1989). However, the apparent links between deprivation and health are based on comparing the measured deprivation in a person's residential area at the time of diagnosis or death; the assumption being that the deprivation score of the current area of residence is an adequate measure of exposure to possible risk (Gatrell, 2002). This ignores the fact that people move so that, for many, the circumstances that had most influence on their health may not have been the same as when their health deteriorated or when they died. Bentham (1988) believes that most studies of associations between diseases and hypothesised causes have paid insufficient attention to the implications of residential migration. Sometimes real environmental causes of disease may be obscured, in other circumstances spurious associations may be produced. While the problems of associating residence at the time of diagnosis with exposure to environmental conditions have been researched (e.g. Mancuso & Sterling, 1974), studies that account for migration are rare (though see, for example, Giggs, Bourke, & Katschinski, 1988; Kliewer, 1992; Riise et al., 1991; Rogerson & Han, 2002; Sabel, Gatrell, Löytönen, Maasilata, & Jokelainen, 2000). Ignoring population mobility is a problem because migration is selective.

At any one time the characteristics of migrants in the UK are different from the population as a whole and the migration process is therefore selective. Migrants differ from the rest of the population in terms of their age, life stage, housing tenure, socio-economic position and educational achievement (Boyle, Halfacree & Robinson, 1998; Buck, Gershuny, Rose, & Scott, 1994; Champion et al., 1998). Importantly, migration is also selective by health status and we know that young adult migrants tend to be healthier than non-migrants, while older migrants tend to be less healthy than their sedentary counterparts (Bentham, 1988; Findlay, 1988; Boyle, Norman, & Rees, 2002; Rogerson & Han, 2002; Boyle & Duke-Williams, forthcoming, 2004).

Of course, migrants are also selective in terms of their choice of destination. Place characteristics have long been acknowledged as important determinants of migration (Walters, 2000) and factors that potentially ‘push’ or ‘pull’ migrants between different places vary with age and stage in the life course (Boyle et al., 1998; Champion et al., 1998). Young adulthood is the peak age for migration when people tend to move into urban areas for education and employment opportunities. In mid-life, moves will usually correspond to changes in employment or housing needs and in later life retirement-related migration may involve moves into care, to be nearer family and friends, or to coastal or semi-rural ‘retirement areas’ (Law & Warnes, 1976; Rogers, 1992). On the whole it seems reasonable to expect that, in terms of aspirations, people will want to move away from more deprived locations and towards less deprived locations.

Bentham (1988) distinguished between three types of health selective migration. First, if sick migrants are able to move away from an area perceived as harmful to health (a deprived location, for example), the effect would be to reduce observed mortality and morbidity rates in migrant origins and to raise rates in the destinations. This would diminish the strength of relationship between health and area deprivation. Even if migration is not selective of individuals by their health status, it still has implications for geographical studies of associations between health and area characteristics. If people have only recently migrated into an area, they may not have been exposed to local conditions long enough for any adverse effect on their health and this will artificially reduce any excess in mortality or morbidity rates in the area. Second, people who are ill may move to be better placed for either formal medical care or informal care provided by their families. Locations with care institutions will attract populations selected for their poor health, even though they may be situated in relatively wealthy places. Theoretically, this will elevate mortality and morbidity rates in areas with institutions whilst migrant origins without care institutions will show apparently favourable levels of health. Third, while some types of migration may be selective of people in poor health, migration in general will be dominated by individuals, particularly young adults (who are the most mobile group), whose health is better than average. The depletion of a relatively healthy proportion of the population would result in higher morbidity and mortality rates at their origins. The converse will be found for areas that are the destinations of relatively healthy migrants.

Dorling, Shaw, and Brimblecombe (2000) ask whether health selective migration can explain geographical inequalities in health and to date there is conflicting evidence regarding the scale over which these effects may operate. As long ago as 1864 Farr noted that migrants from urban to rural areas differed in their health from migrants moving from rural to urban areas. In 1872 Welton pointed out that selective migration could have a profound effect on local mortality rates. Welton showed that younger migrants leaving rural areas to live in cities were healthier than those who remained behind so that origin area mortality rates were raised. At the same time, in-migration to cities of this selectively healthy subgroup had the effect of reducing mortality rates.

Fox and Goldblatt (1982) found that health-related differential migration in England contributed to widening regional differences in health inequalities since people moving away from high mortality areas had lower mortality rates than those moving in and persons moving between ‘healthy’ regions had lower mortality rates than both of these groups. On the other hand, Brimblecombe, Dorling, and Shaw (1999) found that migration made little difference to inter-regional differences in mortality, but at local authority district level the observed differences in mortality were accounted for entirely by migration. Since the majority of migration is short distance and there are wider mortality gaps between small areas within regions than there are between the regions, it would be surprising if migration did not in part contribute to inequalities between areas.

We should also be aware that people can experience changing levels of deprivation through time without moving. Focusing on non-migrants in non-deprived households, Boyle, Norman, and Rees (2004) show that when the places they are living in become more deprived over time, their health suffers. Thus a line of enquiry is whether, over and above this effect on non-migrants, there is an additional effect on ill health and mortality rates that is due to migrants changing their location.

The selective nature of migration is particularly problematic when cross-sectional data are used. In this research an extract from the Office for National Statistics (ONS) England and Wales Longitudinal Study (LS) for the period 1971–1991 has been reaggregated at different time points to create population samples stratified by ward deprivation categories. This approach allows aggregate effects of the changes in individual circumstances to be explored. Specifically, we examine whether healthier persons tend to migrate into less deprived locations and less healthy persons into more deprived locations, thereby exaggerating the apparent relationship between deprivation and health. We do this using both limiting long-term illness (LLTI) in 1991 and all-cause mortality between 1991 and 1999 as the outcome measures. Essentially, this paper seeks to determine if, over the 20-year period from 1971 to 1991, there has been any systematic sorting of healthy and unhealthy people along the gradient of deprivation.

Section snippets

The ONS longitudinal study data for England and Wales

The ONS LS contains linked decennial census and vital events data (e.g. births and deaths) and cancer registrations for a one percent sample of the population of England and Wales2 (Hattersley &

Methods

We calculated indirectly standardised illness and mortality ratios (SIRs and SMRs, respectively) for different combinations of deprivation categories as recorded in 1971 and 1991. Expected morbidity and mortality were calculated using all the LS members present in 1971–1991 as the standard population and the 95% confidence intervals were calculated as:SIRorSMR±1.96×100×ObservedExpected.

We also calculated age-specific illness rates (ASIRs) and mortality rates (ASMRs) and the 95% confidence

Results

According to the Census Local Base Statistics 100% data for England and Wales, 12.2% of people reported LLTI in 1991, while 16% of our sample reported LLTI. This is because the 0–9 and 10–19 cohorts are not included in 1991 and rates of LLTI are considerably lower in the younger age groups. Of the LS members in the extract 12% died between 1991 and 1999 and, of these persons, 49% had reported LLTI in the 1991 Census. In 1971 26% of the sample lived in the most deprived wards (Carstairs quintile

Conclusion

Strong relationships between deprivation and health have regularly been demonstrated but few epidemiological or geographical studies account for the effects of selective migration; this is particularly the case in cross-sectional analyses. If healthier individuals are more likely to migrate away from deprived areas and less healthy individuals more likely to migrate into deprived areas then the aggregate relationship between illness and deprivation and the health inequalities between small

Acknowledgements

The Census data are Crown Copyright. The Longitudinal Study (LS) data extract has been provided by permission of the Office for National Statistics (ONS) through the LS academic user support group at the Centre for Longitudinal Study (CLS) at the Institute of Education, University of London with assistance from Sarah Jones and Kevin Lynch. Angela Donkin (ONS) and Rosemary Creeser (CLS) provided advice in the early stages of the project. Latterly, LS academic user support has been by ‘CeLSIUS’

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