Commentary: Socioeconomic health differentials

David Blane

Department of Social Science and Medicine, Imperial College of Science, Technology and Medicine: Charing Cross, St Dunstan's Road, London W6 8RP, UK. E-mail: d.blane{at}ic.ac.uk

The present issues of the IJE contain eight reports of socioeconomic differences in health. These articles illustrate a number of traditional themes and point to new directions. The papers come from a range of countries—Australia, Denmark, Finland, Italy, The Netherlands, Scotland and Sweden—thereby demonstrating once again1,2 the ubiquity of socioeconomic inequalities in health, at least in the rich countries of the world.

These inequalities are maintained over time against a background of generally improving health. Turrell and Mathers3 report falling mortality rates combined with socioeconomic mortality differentials which widen or narrow somewhat, by age group and gender, but which remain a pervasive characteristic of the population's health. The Black Report4 analysed the combination of falling mortality and maintained differentials as a form of relative deprivation in which the health of the socially disadvantaged, in time, catches up with that of the socially advantaged, by which time however, the health of the advantaged has forged even further ahead. This basic characteristic of health inequalities provides an important clue to their causes. Only some form of social causation, whether behavioural, psychosocial or material, can account simultaneously for both parts of the phenomenon—the falling mortality and the maintained differentials. The potential importance of one form of social causation, namely socioeconomic differences in quality of medical care, is undermined by Middlekoop et al.5 who show that higher case-fatality rates are not the cause of the excess of avoidable mortality in deprived areas.

There is a growing consensus that health inequalities need to be explained, as well as described; and that a life course perspective is required to explain them as biologically plausible phenomena6,7 The papers by Wamala et al.8 and Kaplan et al.9 add to this literature. Wamala et al. report that women exposed to socioeconomic disadvantage in both early life and later life have some four times the risk of developing coronary heart disease, compared to those without disadvantage at either stage of life, and that the effects of each life stage are cumulative, but not interactive. The West of Scotland Collaborative Study found that men's mortality risk increased cumulatively with the proportion of life spent in disadvantaged social positions;6 Wamala et al. demonstrate that this finding can be extended to women. Kaplan et al. report a relationship between adult cognitive function and a characteristic of childhood, namely mother's education, which is independent of its well recognised relationship with the subject's own educational attainment. This association is biologically plausible, in terms of, for example, an educated woman's verbally complex interaction with a young child stimulating the development of its brain; in the same way that the relationship between childhood growth and adult blood pressure can be explained in terms of childhood stress suppressing both physical growth and the brain receptors which control the response to stressors in later life.7

Significant incremental gains in knowledge can be achieved by studies which eliminate possible explanations of earlier findings. The socioeconomic gradients in lung cancer mortality that have been found within each category of tobacco smoking (never, ex and current) could be either an artefact of residual confounding, due to imprecise measurement of smoking exposure, or caused by other factors associated with socioeconomic position, such as exposure to occupational carcinogens or dietary anti-oxidants. Martikainen et al.10 have tested the former possibility and show that little effect follows from greatly increasing the precision with which smoking exposure is measured. The paper by Hart et al.11 is relevant to the latter possibility; poor lung health, deprivation and poor socioeconomic conditions throughout life are identified as the non-tobacco factors which contribute to social class differences in lung cancer mortality.

Two papers address socioeconomic differences in the prevalence of risk factors for coronary heart disease. Suadicani et al.12 show that the ability of these risk factors to explain socioeconomic differences in ischaemic heart disease mortality attenuates with length of follow-up. Episodic job-related heavy physical labour is found to be the most powerful single explanatory factor at each stage of follow-up; it also interacts strongly with tobacco smoking. In view of speculative claims that changes in the social class distribution of smoking are driving contemporary changes in health inequalities, it is interesting that current smoking, on its own, proves a rather weak predictor. Ferrario et al.13 examine in more detail contemporary risk factor change. No consistent pattern emerges and, when these changes are examined by socioeconomic group, the pattern becomes even more complex and inconsistent. Similar results have been described for England14 and, taken together, suggest that current risk factor change may not be able to explain concurrent changes in the socioeconomic distribution of mortality. One possible explanation may be that risk factors take time to affect organ pathology. If such lag times are to amount to more than post hoc rationalisations, however, their duration needs to be specified in advance and any variation in duration, by gender and country for example, predicted and explained.

The eight papers in the present issue, when read together, raise important issues about the measurement of socioeconomic position. Physicians are familiar with the idea that height and weight are distinct dimensions of physical size; and that measuring them separately can be instructive. Medicine might benefit from applying a similar precision to social phenomena. The distinction between social class and social status is ancient (‘Classes are stratified according to their relations to the production and acquisition of goods; whereas status groups are stratified according to the principles of their consumption of goods, as represented by special styles of life’15), although only recently has its explicit application to epidemiology been advocated.16 The eight papers use several dimensions of socioeconomic position—deprivation, education, class, status— somewhat interchangeably and, occasionally, in the same paper. This is a shame, because different dimensions and their appropriate measures may help to identify different aetiological pathways.17 It is consistent with this point of view, for example, that the distinction between general socioeconomic position and, specifically, education allows Kaplan et al.9 to identify a biologically plausible pathway between a mother's educational level and the adult cognitive functioning of her offspring.

Among the reasons why such efforts are important are two related to policy. The Black Report4 helped to keep health inequalities on the political agenda in Britain and produced, in time, the Acheson Inquiry18 and government commitment to their reduction. The work reported in the present issue demonstrates that a similar process is under way internationally. Second, efforts to reduce health inequalities should benefit from an appropriate knowledge base on which to form policy; for example, should early life be prioritized, or instead, critical transitions during the whole life course? The way we answer such questions may influence policy in the coming decades.

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