Income and health: what is the nature of the relationship?
Introduction
The association between socio-economic position and mortality has been extensively investigated in the UK and elsewhere (Davey Smith et al., 1990a; Kunst and Mackenbach, 1994). Due to the scarcity of routine data on morbidity, the socio-economic distribution of ill-health has been less extensively investigated, although the same general pattern of less favourable outcomes amongst those in worse socio-economic circumstances is seen (Marmot et al., 1991; Eachus et al., 1996). In the UK, Registrar General's occupational social class has been the most frequently utilised socio-economic measure while, in the US, education and income are more commonly used. This reflects differing traditions of routine data collection and perhaps differences in the ways in which the factors underlying the uneven socio-economic distribution of sickness and death are conceptualised (Krieger and Fee, 1994). There may, however, be reasons why different relationships with health measures will be seen when different socio-economic measures are used. For example, income and occupation can be altered by poor health in adulthood, leading to some degree of reverse causation between socio-economic position and health, while health status in adulthood can only rarely influence the level of achieved education. It is therefore important to investigate the associations between health and different socio-economic measures, with income having been poorly studied in this regard in the UK.
In studies from the US, household income and median income of inhabitants of the residential area have been strongly associated with mortality risk (Sorlie et al., 1995; Davey Smith et al., 1996a, Davey Smith et al., 1996b). In the largest study of household income there is a suggestion of a flattening, or even reversal, of the income and mortality relationship at the high end of the income spectrum (Backlund et al., 1996) while this was less evident in the largest study of income of area of residence (Davey Smith et al., 1996a, Davey Smith et al., 1996b). The latter finding is consistent with a considerable body of data demonstrating that socio-economic differentials in health persist across the whole socio-economic spectrum (Davey Smith et al., 1990b, Davey Smith et al., 1994, Davey Smith et al., 1996a, Davey Smith et al., 1996b; Marmot et al., 1991; Ford et al., 1994; Ecob, 1996; Der et al., submitted for publication). The finally graded association between socio-economic position and health is an important contributor to the formulation of explanations for the existence of health inequalities (Davey Smith et al., 1994; Marmot et al., 1995). The investigation of the association between particular socio-economic indicators and a variety of components of health can contribute to understanding in this area. We therefore undertook a detailed exploration of the exact shape of the relationship between household income and several health outcomes, both before and after adjustment for other socio-economic indicators, in a representative sample of adults in England, Wales and Scotland.
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Methods
This analysis is of data from the first wave of the Health and Lifestyle Survey (HALS) (Cox et al., 1987), a national survey of adults in England, Wales and Scotland in 1984–1985. This survey was chosen in preference to other national surveys, such as General Household Survey (GHS, 1997) which has larger sample size and, being annual, is more recent and has better data on income, because of our desire to examine a variety of measures of health, both self-rated and physically measured, which are
Results
The relationship of each health measure to income both before and after social controls is shown in Fig. 1. The graphs are for males in the age group 40–64 except for waist–hip ratio, where females aged 40–64 are plotted. These are the age and sex groups for which, when the relationship to income varies by age or sex, the strongest relationships within working ages (considered as the difference in health measure between the 90th and the 10th percentile of the income distribution) are found.
Income and health
This paper has reported on the relationship of income to morbidity in a national study both before and after controlling for a range of socio-economic variables. Up to now, little has been known about the residual relationship of income to morbidity after controlling for other socio-economic variables. In the UK, this is in large part due to the general omission of income at the individual level from large surveys and the census. The relationship of all measures of health examined, both
Acknowledgements
We would particularly like to acknowledge the advice of Geoff Der in connection with the production of figures in this paper and of Sally Macintyre, Geoff Der and Graeme Ford for comments on earlier drafts; and to thank three referees for useful comments.
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