Cause-specific inequalities in mortality in Scotland: two decades of change. A population-based study

Differentials in mortality rates linked to socio-economic status have increased in several western European countries over recent decades [1]. Inequalities between geographically defined areas increased in Britain between 1979 and 1998 and were greater in Scotland than in England and Wales [2] where recent trends have provided cause for concern. Death rates for men aged 20–24 increased during the 1980s, a rise that broadened to include men aged between 20 and 34 during the 1990s; the causes contributing to these increases included suicide, drugs, alcohol and violence [3]. Their relationship to socio-economic status is unknown.

Recent (although disputed [4]) estimates of social differentials in mortality in England and Wales suggest that about half is attributable to smoking-related deaths [5]; 14% of the mortality differential in Finland is similarly attributed to alcohol-related causes [6]. The implication of these observations is that relatively straightforward social patterning provides an explanation for socio-economic differences in death rates – and, thus, the public health responses to them. The difficulty with this view is that it fails to propose an adequate explanation for the social structures and processes that underlie such outcomes of health-related behaviours.

In this paper we describe age-specific mortality rates for men and women in Scotland between 1981 and 2001 and consider the ways in which the relative importance of selected causes of death has changed. In Scotland, there are important regional differences in the distribution of social disadvantage and so the question of the extent to which unequal death rates are associated with particular localities rather than socio-economic status is also relevant. In a further analysis (for 2000–02) we describe the relative contribution of selected causes of death to inequalities at different ages.

The data comprised death records for 1980–82, 1991–92, and 2000–02 and mid-year population estimates for 1981, 1991, 2002 provided by the General Register Office (Scotland). The information abstracted from the death records included age, sex, cause of death, and postcode sector of residence; population estimates included age, sex and area. In 2001, there were 1010 postcode sectors or part-sectors in Scotland with an average population of 5012. The number of sectors and their mean population varied slightly between time-points. The use of postcode sectors allowed a regional analysis of mortality and enabled us to divide Scotland into seven major regional groupings [7].

Each postcode sector was assigned a Carstairs Score indicative of its relative social deprivation [8‐10]. These scores were derived from measures of overcrowding, male unemployment, households without a car and low social class at each of the three Censuses; they were then divided into seven deprivation categories (DepCats) for that Census. Correlation coefficients for the DepCats from the three Censuses were greater than 95%. At each Census, approximately 6% of the Scottish population lived in areas described as DepCat 1 (the least deprived) and 7% in the most deprived (DepCat 7). A change to some postcode sectors during 1990 meant that we were unable to attach deprivation scores to all deaths for this year and resulted in the restriction of this middle period to two years (1991–92) instead of three.

A second analysis employed a more recent categorisation of the socio-economic status of small areas. This was the income domain of the Scottish Index of Multiple Deprivation (SIMD) for 2004 which comprises eight measures from 2001 and 2002 relating to the receipt of social security benefits and tax credits [11]. The much smaller areas (data zones) for which this measure is provided had a mean population of 778; there were 6505 such zones. The advantage of this measure is that it permits a more detailed analysis of the contribution of particular causes of death to age-specific inequalities; however, it was not possible to analyse deaths in earlier years using this measure. The slope index of inequality (SII) was calculated for each cause and age group across quintiles of the SIMD income score. Division of the SII by national all-cause mortality for that age group then gives a relative measure of inequality [12, 13].

All age standardised rates, separately for males and females, were calculated by direct standardisation to the European standard population. Causes of death were chosen to reflect changing patterns of mortality and are not comprehensive.

To a large extent, the general decline in Scottish death rates over the past two decades can be attributed to reductions in what are usually regarded as the major causes of death – chiefly, ischaemic heart disease, cerebrovascular disease and, to a lesser degree, malignant neoplasms. These reductions in mortality have been greatest in socially advantaged groups in the population but are also apparent in more deprived localities and so the question of the extent to which persisting inequalities simply represent a lag for socially disadvantaged groups within a more general process of decline becomes pertinent. The argument that this is not so is supported by two principal considerations: the first, and more disturbing, is the observation that inequalities are increasing for many causes and are greatest at younger ages with the substitution of "newer" causes (suicide, drugs, alcohol and assault) in the most deprived neighbourhoods. The second is that the decline in death rates in different Carstairs categories is inconsistent: as an illustration, the IHD death rate in DepCat 1 reduced by 73% between 1980–82 and 2000–02 and that for DepCat 2 by 70%. The corresponding reduction for DepCat 7 was 36%. Additional support for the view that these differences are underpinned by unequal social circumstances is found in the extent to which the different rates seen in each DepCat can explain differences in death rates between different parts of Scotland. It is clear from Table 4 that for the most part mortality rates in the Clydeside conurbation are not so different from those for the general population within each deprivation band. This means that the considerable excess mortality of the Clydeside region – mortality in the region was 29% above the Scottish rate in 2000–02 [7] – derives from the greater prevalence of social disadvantage in Clydeside. The proportion of the population of the region that lives in relative deprivation has changed little over time, and so the fact that the excess mortality in Clydeside has increased from 17% in 1980–82 and 22% in 1991–92 is likely to be due to the increasing differences in mortality rates between the deprivation groups.

Our findings concur with the increasing socioeconomic inequalities seen in other countries up to the end of the 1990s based on individual measures of education [1, 15, 16] or socioeconomic status derived from occupation [1, 17]. Such increases were for the most part attributable to changes in the rates of cardiovascular disease, with declines being steepest among the most privileged groups. There are particularly pertinent comparisons to be drawn between the results we present for Scotland and the detailed published analysis of mortality rates in Estonia, where declining all cause mortality rates among men aged 20–39 with University education were offset by increases among men with lesser education resulting in an overall increase of 25% in this age group between 1989 and 2000, and where differential increases in alcohol-related mortality contributed significantly to the widening inequalities [15].

Results based on individual socioeconomic status (not presented here) were inconclusive for three principal reasons. Firstly, there has been a change in the coding of social class between 1991 and 2001 from the Registrar General's Social Class to the National Statistics Socioeconomic Classification (NS-SEC) making comparisons over time difficult. Secondly, a large proportion of deaths and of the population at certain ages cannot be allocated to a social class and it is difficult to understand the "not classified" category. Finally, there was a considerable mismatch between the proportion of the population not classified on death records and in the general population, with the problem being more pronounced at younger and older ages and for women [7].

The area deprivation measures employed indicate relative deprivation, identifying proportions of the population living in the most deprived areas at each time. This raises the question as to whether the finding of increasing inequalities between the different deprivation categories truly reflects increasing inequalities between equivalent groups or whether selective population migration has led to an increased polarisation of Scottish society, with the deprived population becoming more deprived over time. Although it is impossible to answer this with certainty, we can find clues about the population change by considering the characteristics that comprise the Carstairs index [9, 10]. Between 1991 and 2001 male unemployment fell from 35% to 19% in DepCat 7 compared to a decline from 13% to 8% in Scotland as a whole. This implies a slightly greater relative decline in unemployment in DepCat 7 than was seen in the rest of the country. However, the proportion of low social class fell from 21% to 18% throughout Scotland whilst the decline in DepCat 7 was just from 33% to 32%. With the declines in overcrowding and households without a car being approximately equivalent in DepCat 7 and the rest of Scotland an increase in deprivation in DepCat 7 is not obvious.

In 2000 the system of coding of deaths in Scotland changed from the 9^th to the 10^th revision of the International Classification of Diseases (ICD-9 to ICD-10). Such a change may influence the way in which particular deaths are coded with implications for trends in specific causes of death. The trends may also be influenced by incidental changes in coding rules applied in Scotland and in changes in the reporting of causes of death on death certificates. Although it is difficult to quantify the exact impact of such changes, recent European comparisons suggested that whilst a discontinuity associated with a change in ICD coding was discernible in about 10% of cases, the impact of the change in classification from ICD-9 to ICD-10 was small compared to the magnitude of the changes in death rates and in inequalities associated with particular causes shown in this paper [18]. Similarly, in the United States changes in ICD classification system were found to have minimal impact on trends [19]. However, these reports were restricted to broad groups of causes of death or to the causes most common among older age groups. So what of the other causes considered in this paper? It has been suggested that the rise in deaths due to liver cirrhosis were evident over a long period of time and could not be artefacts of the coding change [20]. Similarly, trends in deaths due to suicide in the United Kingdom [21] and drug- and alcohol-related deaths in England and Wales [22, 23] did not show marked differences following the coding changes with the exception of mental and behavioural disorders due to the use of alcohol which showed a decline of 11% among women. A bridge-coding analysis in Scotland identified few changes following the introduction of ICD-10 with the exception of a decrease in the number of deaths assigned to the alcohol abuse code [24]. The lack of change associated with the new coding system does not mean that there have not been changes in coding rules that have led to more gradual shifts in the coding of deaths. If the coding of particular causes has changed over time then it is possible that the changes may differentially affect one population group more than another leading to apparent changes in inequalities. However, such coding changes will not, of course, influence the figures for all cause mortality. So widening inequalities in all cause mortality must reflect increasing inequalities in certain causes; any uncertainty is over the extent of the increase for individual causes.

Without denying the importance of specific public health interventions – such as those directed at smoking and alcohol consumption – it is difficult to escape the conclusion that the inequalities we describe have more fundamental origins in lifestyles determined by poverty. The improvement in IHD mortality over 20 years in the more affluent areas implies an ability to adopt healthier life-styles and behaviours. The reverse of this coin, characterised by a growing number of young deaths from essentially negative lifestyles, is evidence of a need for social policies which have a more direct influence on poverty and its correlates.

The problems we describe and the question as to how to formulate policy to address health inequalities are not new and are not limited to Scotland. There is little evidence available regarding the effectiveness of policies [25] or interventions [26] to inform the creation of such policies. The lack of evidence does not mean that the issue has not been addressed; the Independent Inquiry into Inequalities in Health in the UK was sufficiently persuaded by the likelihood of a link between low income and poor health to say that 'without a shift of resources to the less well off ... little will be accomplished in terms of a reduction in health inequalities by interventions addressing particular "downstream" influences' and to recommend policies to reduce income inequalities [27]. The WHO Commission on Social Determinants of Health will recommend policies designed to improve the health of the world's most vulnerable people; they take the view that 'if the major determinants of health are social, so must be the remedies' [28]. It is encouraging that the Scottish Executive has recognised these issues [29] but the scope of the devolution settlement is limited in terms of its ability to tackle the fundamentals underlying deprivation. Tackling health inequalities and their causes is not solely the preserve of health policy [28] and there is a case for more effective action from central governments.