Here we present data for the whole of England that demonstrate that socioeconomic inequality between neighbourhoods leads to poorer health: the population of a small area (comprising a population of approximately 1500 persons, n=32482) suffered greater ill-health if it was surrounded by areas of lower deprivation. We further demonstrate that these links are stonger than the previously described relationship between neighbourhood inequality and mortality, in a study that used the same methodology . Allender et al. carried out an analysis using larger geographical areas within England (wards: population ~6500, n=7927) , and showed that deprivation inequality was harmful for population health (as measured by rates of mortality from coronary heart disease). However, relative deprivation had a relatively weak effect and did not improve the predictive power of their models. Our study confirmed the association at relatively smaller areas and also identified the direction of the influence of the deprivation differential on health. In addition, we found that the deprivation differential (relative deprivation) did improve the predictive power across the whole dataset (e.g. from 43% to 53% for ‘not good health’).
The fact that deprivation inequality is linked to ill-health has been explained by two competing hypotheses, which have generated much controversy [15, 30]. The two hypotheses provided two distinct predictions for the direction of the relationship between neighbourhood inequalities and health. We did not find support for the neo-materialistic hypothesis, which predicted that poorer areas surrounded by greater affluence would have improved health (as a result of better infrastructure). The psychosocial model  predicts poorer health in a more deprived area surrounded by relatively less deprivation than would be found if the same area was surrounded by equivalent deprivation: which was indeed what our results have shown. Socioeconomic inequality is hypothesised to have a psychological and emotional impact which can lead to deterioration in physical health. The proponents of the psychosocial model have articulated a plausible biological pathway: the response to psychological stress involves the release of hormones (e.g. glucocorticoids) by the neural nerve and endocrine system, which circulate in blood system . This stress response is beneficial in the short-term (e.g., glucocorticoid secretion promotes the metabolism of protein and lipids to carbohydrates to give the body energy). However, the long-term secretion of hormones under psychological stress (i.e., glucocorticoid excess) leads to hypertension (high blood pressure) and cardiovascular disease . Here we suggest that social comparison is not only harmful to health in a wider social context, but it also happens between neighbourhoods.
We previously demonstrated the same between-area inequality effect using the more objective measure of mortality . We argue that the two self-reported health variables used in this study could represent an intermediate step between psychosocial stress and an objective adverse outcome such as mortality, supported by the fact that self-reported health was influenced to a greater extent by the surrounding neighbourhood deprivation than was mortality. For mortality, when the data were segmented, the effect of the target area deprivation was larger in every population cohort (most deprived third: 2.5 times greater; middle deprived: 2 times greater), with the effect of the deprivation differential approaching equality within the least deprived cohorts (least deprived: 1.2 times greater) . In this paper we show that the difference in deprivation between one area and its neighbours has an equally strong effect on self-reported health as the deprivation of the area itself in the middle third and the least deprived third of the areas. This observation fits with the notion of poor self-reported health representing an intermediate (and more senstive) response to relative deprivation.
The fact that both mortality and morbidity show similar relationships with the target area deprivation and the deprivation differential is not surprising as self-reported morbidity is highly correlated with mortality: for example, for England as a whole, there is a strong correlation between self-reported ‘not good health’ and all-cause mortality (r=0.86, n=352 English local authorities) . Measures of self-reported health are also strongly related to objective measures of morbidity [34, 35]. It has also been shown that a response of ‘good health’ on this single question is more strongly linked to physical health than to the mental or social health domains of the SF-36 health survey . However, it is also plausible that such self-assessment could additionally be influenced by psychological factors. If a relatively affluent area is situated within a wider area that is even more affluent, then individuals might feel less positively about their status and possibly also their health than they would if they lived in an area surrounded by equivalent affluence. In this case, psychosocial effects may particularly influence the perception of self-rated health compared to other, less subjective, measures of health. Although self-reported health can be criticised for being too subjective [33, 37], it is precisely this element of subjectivity that might explain the stronger effect of relative deprivation on self-reported health status shown in this paper compared to the previously demonstrated relationship with mortality . This is consistent with the notion that psychosocial effects mediate the relationship between health and the deprivation of surrounding areas.
Areas at the less deprived end of the spectrum have health that is better on average. However, there is likely to be a non-linear relationship between deprivation and health such that reductions in deprivation have less impact on health for the least deprived areas . Thus, at the lower end of the deprivation spectrum (i.e. in more affluent areas) there may be more capacity for average health at an area level to vary in response to a source of inequality that is relatively removed (i.e. comparisons between neighbouring areas of population size 1500, with an average distance between them of 6 km). In the more deprived areas, health is already depressed by the poverty of the area itself. Moreover, the psychological effects of relative deprivation (or relative affluence) may differ depending on whether a person is poor or affluent. There is some evidence to support this: affluent people living in poorer areas rated themselves as higher on the social ladder than equally affluent people living in affluent neighbourhoods, while poorer people living in affluent areas rated themselves more highly than equally poor people living in poor areas .
Interestingly, the only other study that we could find that uses a similar methodology  finds the reverse relationship between deprivation differential and ill-health, in this case the incidence of type 2 diabetes. Cox et al.’s study was also set up as a test of the two hypotheses, and therefore found support for the neo-materialistic approach. However, their study used smaller geographical areas (Output areas, around 150 households). The outcome measure used (diabetes) relied on a diagnostic resource that may have been more available in surrounding wealthier areas, and it is not known to what extent their findings reflect the underlying incidence of diabetes or the likelihood of diagnosing the condition. Neither the outcome measures used in this study (self-reported ill-health derived from the census) nor our previous study (using routine data on mortality) relied on the availability of any services for diagnosis.
For the affluent P2 People & Places categories of ‘Mature Oaks’, ‘Country Orchards’, ‘Blossoming Families’ and ‘Rooted Households’, a one unit increase in the deprivation differential was as significant for health as a unit increase in the deprivation of the area itself. Mature Oaks and Country Orchards are relatively wealthy, with a predominance of retired couples, while Blossoming Families tend to be composed of younger families with children (see appendix for description of P2 People & Places classification). In contrast, at the other end of the categorisation, the effect of the deprivation differential on self-reported health status did not always follow the order of average deprivation of the groups. The medium deprived groups, New Starters and Qualified Metropolitans, showed the weakest (but still significant) effect of the deprivation differential. This finding was similar to that observed for mortality . Previous research has shown that New Starters and Qualified Metropolitans are outliers in several analyses of ill-health and have more risk indicators than would be predicted by the deprivation alone . New Starters have higher rates of smoking-related and alcohol-related conditions, mental health conditions and all cause mortality. In contrast, Qualified Metropolitans have lower than expected rates across many indicators (e.g. accidents, asthma, coronary heart disease). The features of these groups that make them outliers across a range of other indicators may be the same as those that lead to them having a weaker relationship with the deprivation differential than would be expected. However, the mechanism of this is unknown.
Several limitations to this study should be acknowledged. First, the deprivation indices and health indicators were generated from census data and then applied at an aggregate level, which raises the possibility of the ‘ecological fallacy’ whereby the average characteristics of the population are assumed to represent the individial [41, 42]. This pooling of populations for analysis would tend to towards the null rather than to spurious significance. Second, the morbidity data (‘not good health’ and LLTI) used the readily available public dataset and were not standardised by age. However, self-reported health differs by age, for example, old people are more likely to report LLTI [43, 44]. Thus, the evaluation of deprivation differential on morbidity still needs to be further validated. However, our previous study using mortality as the outcome variable did use age-standardised data and found a similar relationship between the deprivation differential and mortality , suggesting that the patterns are robust. Third, we were unable to take migration into account, which might bias our results. It is known that healthy, affluent people are more likely to move away from less favourable environments . In a Dutch study, those with higher education levels were prone to move out of relatively poor areas . In our study, the morbidity data were collected in 2001, while The IMD index was released in 2007 (using data from 2001 to 2005). It is possible that migration caused the population to change between the two time points. Such migration could influence the illness rate and deprivation status of the origins and destinations, and then confound the relationship between health and deprivation [47, 48]. However, for this to bias the results in favour of the observed deprivation differential effect, the migration would have to happen at a greater rate across borders with a higher deprivation differential. Migration effects may be more likely to bias the results towards the null.