Imperial College London, London W6 8RP, UK. E-mail: d.blane{at}imperial.ac.uk
The paper1 by Regidor and his colleagues, which is published in the current issue of International Journal of Epidemiology, is more or less contemporaneous with broadly similar work from Finland2 and Norway.3 These analyses, taken together, have prompted me to examine a largely unnoticed background assumption to life course analysis. Pioneering work on the West of Scotland Collaborative Study46 sparked a widespread interest in the independent and accumulative influence on mortality risk of socioeconomic position at various stages in the life course. Subsequent analyses in other data sets have failed sometimes to replicate these Collaborative Study results. The present commentary examines one possible explanation for these variable results; namely, that the nature of life course socioeconomic position can be obscured by the transition from a predominantly rural society of subsistence farmers to an urban society of wage workers.
Victor Hawthorne and Charles Gillis, when they designed in the early 1960s the questionnaire for what became known as the West of Scotland Collaborative Study, included three questions on occupation; occupation of father during the respondent's childhood, the respondent's first significant occupation after starting paid employment, and the respondent's occupation at study screening. These occupations were coded to the UK Registrar General's classification, to give social class during childhood, early adulthood, and middle age. When, some quarter of a century later, Charles Gillis and George Davey Smith laid plans to analyse the mortality follow-up, they were constrained by the ideas then current. As indicated by the study title (Social status in early life, social mobility, health behaviours and cardiovascular disease mortality risk), they thought to use the three social class positions to examine social mobility; and were influenced also by David Barker's ideas, which at the time were starting to receive support from individual-level data. With two notable exceptions,7,8 there was little in the literature to guide George Davey Smith when he used social class position during childhood, early adulthood, and middle age to construct a scale, ranging from 0 to 3, of the number of times a person had occupied a manual social class position. What struck me, when I saw the preliminary results, was that this accumulative measure of the proportion of life spent in manual social class positions predicted not only the subsequent mortality risk but also physiological and clinical status at the time of screening (respectively: systolic and diastolic blood pressure, serum cholesterol concentration, body mass index and forced expiratory volume in one second; and angina and bronchitis).5
These relationships have been examined subsequently in other data sets, with, as noted earlier, variable results. Much of this variability could be due to differences in geographical and historical context. The West of Scotland Collaborative Study was conducted in Glasgow and its adjacent urban areas, at a time when the main migration into Glasgow of subsistence farmers from the Scottish Highlands and the countryside of Eire had been completed some 50 years earlier. Its population, consequently, had been urbanized for at least two generations. Urbanization is a more recent phenomenon in many other European countries, including Spain, Finland, and Norway, where the parents of many urban adults are or were rural dwellers, often small subsistence farmers. Most socioeconomic classifications, such as UK Registrar General's social classes, are concerned primarily with the occupations of urban, industrial populations; and do not include easily rural subsistence farmers. Land rights and inheritance laws vary from country to country, but small subsistence farmers (peasants, crofters, and so forth) in most countries have formal or traditional ownership of, or claim on, their land, whose size often shrinks over the generations, due to division at inheritance. Formally, then, these individuals are landowners; while, in practice, they work long hours of heavy manual labour for a low standard of living. Integrating these processes into a life course analysis, in a way which makes sense, requires clarity about two issues. First, how to include subsistence farmers into classifications of socioeconomic position which are primarily urban and industrial. Second, what does one intend a socioeconomic position to indicateformal ownership or standard of living and working conditions?
Fortunately, life course epidemiology is not the first branch of knowledge to face these problems. Robert Erikson and John Goldthorpe, in their classic study of comparative social mobility in Europe,9 were forced by the large rural populations of countries such as Poland and Hungary to consider how to accommodate small farmers within their Erikson-Goldthorpe-Portocarero (EGP) social schema. Within the full version of the EGP schema, farmers and smallholders and other self-employed workers in primary production (class IVc) are distinguished from agricultural and other workers in primary production (class VIIb). In the EGP's collapsed versions, class VIIb is aggregated into manual workers, while class IVc remains a distinct grouping of farm workers. Whether or not life course epidemiology adopts the Erikson-Goldthorpe method, it does need to address the issue of urbanization; and find a solution to the methodological and substantive problems it poses. The extension of life course epidemiology to countries where urbanization is still under way will require even more account of such processes and more imaginative solutions. In one life course study, for example, childhood socioeconomic position in Brazil was measured in terms of the number of hectares of land owned by the father and whether the parental home had been built of bricks.10 These are among the challenges facing the coming generation of post-doctoral life course researchers.
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2 Pensola T, Martikainen P. Cumulative social class and mortality from various causes in young men. J Epidemiol Community Health 2003;57:74551.
3 Naess O, Claussen B. Life-course influences on social inequality in adult mortality risk: a review. Norsk Epidemiologi 2002;12:2731.
4 Blane D, Hart CL, Davey Smith G, Gillis CR, Hole DJ, Hawthorne VM. The association of cardiovascular disease risk factors with socioeconomic position during childhood and during adulthood. BMJ 1996;313:143438.
5 Davey Smith G, Hart C, Blane D, Gillis G, Hawthorne V. Lifetime socioeconomic position and mortality: prospective observational study. BMJ 1997;314:54752.
6 Davey Smith G, Hart C, Blane D, Hole D. Adverse socioeconomic conditions in childhood and cause-specific adult mortality: The Collaborative Study. BMJ 1998;316:163135.
7 Mare R. Socio-economic careers and differential mortality among older men in the United States of America. In: Vallin J, D'Souza S, Palloni A (eds). Measurement and Analysis of Mortality: New Approaches. Oxford: Clarendon Press, 1990, pp. 36287.
8 Salhi M, Caselli G, Duchene J, Egidi V, Santini A, Thiltges E, Wunch G. Assessing mortality differentials using life histories: a method and applications. In: Lopez A, Caselli G, Valkonen T (eds). Adult Mortality in Developed Countries: from Description to Explanation. Oxford: Clarendon Press, 1995.
9 Erikson R, Goldthorpe J. The Constant Flux: a Study of Class Mobility in Industrial Societies. Oxford: Clarendon Press, 1993.
10 Nicolau B. The Relationship between Early Life and Life Course and Oral Health Status. PhD University of London 2001.