© 2006 American Public Health Association DOI: 10.2105/AJPH.2004.060970
Marilyn Winkleby and David Ahn are with the Stanford Prevention Research Center, Stanford University School of Medicine, Stanford, Calif. Catherine Cubbin is with the Center on Social Disparities in Health, Department of Family and Community Medicine, University of California, San Francisco, and with the Population Research Center, University of Texas, Austin. Correspondence: Requests for reprints should be sent to Marilyn Winkleby, Stanford Prevention Research Center, Stanford University School of Medicine, 211 Quarry Road, Stanford, CA 943055705 (e-mail: winkleby{at}stanford.edu).
Objective. We examined whether the influence of neighborhood-level socioeconomic status (SES) on mortality differed by individual-level SES. Methods. We used a population-based, mortality follow-up study of 4476 women and 3721 men, who were predominately non-HIspanic White and aged 2574 years at baseline, from 82 neighborhoods in 4 California cities. Participants were surveyed between 1979 and 1990, and were followed until December 31, 2002 (1148 deaths; mean follow-up time 17.4 years). Neighborhood SES was defined by 5 census variables and was divided into 3 levels. Individual SES was defined by a composite of educational level and household income and was divided into tertiles. Results. Death rates among women of low SES were highest in high-SES neighborhoods (1907/100000 person-years), lower in moderate-SES neighborhoods (1323), and lowest in low-SES neighborhoods (1128). Similar to women, rates among men of low SES were 1928, 1646, and 1590 in high-, moderate-, and low-SES neighborhoods, respectively. Differences were not explained by individual-level baseline risk factors. Conclusion. The disparities in mortality by neighborhood of residence among women and men of low SES demonstrate that they do not benefit from the higher quality of resources and knowledge generally associated with neighborhoods that have higher SES.
An established body of contextual studies in the United States has demonstrated that neighborhood indicators of socioeconomic status (SES) predict individual mortality.19 Most studies show significant, but modest neighborhood effects after they account for individual SES and other factors. These studies have focused on the main effect of neighborhood SES on mortality. However, several studies in the United States and elsewhere have examined the cross-level interaction between individual and neighborhood SES on mortality,6,8,10,11 which provides the opportunity to explore whether neighborhood effects are different for women and men of low and high SES. For example, adults of low SES in high-SES neighborhoods might experience a lower risk of dying than adults of low SES in low-SES neighborhoods, because they benefit from the collective resources in their neighborhoods.6,1215 Alternatively, adults of low SES in high-SES neighborhoods might experience a higher risk of dying because of relative deprivation, low relative social standing, or both.1618 Our study adds to previous work by using a population-based sample, extended mortality follow-up (mean 17.4 years), comprehensive survey and medical data, and geocoded data about neighborhood goods and services. Two study questions were examined: (1) does neighborhood SES exert a different effect on risk of dying for women and men of high, moderate, and low SES, and (2) are any differences explained by individual baseline sociodemographic characteristics, health behaviors, risk factors, health status, causes of death, and proximity to goods and services near participants homes.
Design and Sample Data are from the Stanford Heart Disease Prevention Program, a cardiovascular disease intervention study.1921 Participants were drawn from 2 treatment (Monterey, Salinas) and 2 control (Modesto, San Luis Obispo) cities in Northern California with populations that ranged from 35 000 to 145 000 residents in 1980. Our analysis included adults, aged 2574 years, who were English- or Spanish-speaking and who participated in 1 of 5 separate cross-sectional surveys that were conducted from 1979 to 1990. For each survey, the sampling unit was the household. All dwellings were enumerated and households were randomly selected from directories that contained a list of dwellings. To avoid the possibility of clustering of risk factors by household, 1 woman, 1 man, or 1 of each was randomly selected from each household, and results were stratified by gender. Sample sizes by survey were 1603 in survey 1 (19791980), 1652 in survey 2 (19811982), 1763 in survey 3 (19831984), 1682 in survey 4 (19851986), and 1719 in survey 5 (19891990). Because few or no significant changes in risk factors21,22 and morbidity or mortality23 were found between treatment and control cities, all cities were combined.
Nurses and laboratory technicians collected survey and medical data at centers in the 4 cities. Response rates for the 5 surveys were 65%, 69%, 65%, 56%, and 61%, respectively. All participants were able to attend a 2-hour clinic visit at baseline that included a physical activity assessment using a stationary bicycle. A questionnaire was completed for eligible individuals who declined to participate in the study (75% response). There were no significant differences between respondents and nonrespondents for age, gender, or body mass index (P
Death Certificate Match
Neighborhood Definition Although data were clustered by neighborhood, the intraclass correlation was very low (.007), which indicated that one persons mortality was unlikely to affect another persons mortality in the same neighborhood. The intraclass correlation was computed according to the formula provided for a multilevel logistic model.26
Address Geocoding
Neighborhood-Level Socioeconomic Status
Individual-Level Measures
Analytic Approach Survival curves, stratified by individual SES and neighborhood SES, were estimated with time to death or censoring at December 31, 2002,37 adjusting for age as a continuous variable using the SAS PHREG procedure.38 A Cox proportional hazards model was used to test for differences between survival curves. This is the standard model for analyzing survival curves,39 and previous neighborhood studies have used similar approaches.4,7,24,25 The Cox model included age as a continuous variable, centered at mean age,40,41 individual SES and neighborhood SES (both coded 0.5, 0, +0.5 on an ordinal scale), and an interaction term between individual SES and neighborhood SES. Separate models were estimated for women and men. The model was repeated, and 6 baseline risk factors were added in as covariates (obesity, smoking, hypertension, hypercholesterolemia, physical inactivity, and alcohol intake) to test whether these individual risk factors explained differences in survival.
There were 4476 women and 3721 men in the sample. Approximately 83% were non-Hispanic Whites, 11% were Hispanic, and 6% were of other racial/ethnic backgrounds. Sixty-nine percent were married and 78% had lived in their community 5 years or longer. Women and men from all education and income levels were adequately represented. By the end of follow-up, there were 575 deaths among women and 573 deaths among men. Each of the 5 census variables in the neighborhood SES index showed substantial variability across neighborhood SES; high-SES neighborhoods had the most advantageous characteristics. For example, in 1980, the mean percentages of study subjects who had a high-school education or more in high-SES and low-SES neighborhoods were 83% and 48%, respectively; mean annual family income was $24,000 and $14,000, respectively. A similar pattern was evident for each of the variables in 1980 and 1990 and in each of the 4 cities.
Death rates for women and men who had moderate and high individual SES showed no clear pattern by neighborhood SES but overall were substantially lower than those for women and men of low SES (Table 1
Age-adjusted hazard ratios followed a similar pattern. The risk of death, as indicated by the hazard ratios for women and men of low SES in neighborhoods with high SES, was 70% and 74% higher, respectively, than the risk of death for women and men of high SES in neighborhoods with high SES (Table 1
The survival curves that examined at what time point mortality differences were first apparent showed that all women and men who had low SES fared poorly as the curves began to separate (Figure 1
At baseline, individuals who had low SES in high-SES neighborhoods had significantly higher levels of education, higher median household income (men only), lower prevalence of obesity (women only), and higher levels of cardiovascular disease knowledge, although they also had higher mean age and higher prevalence of hypertension (men only) than did individuals who had low SES in low-SES neighborhoods (P <.05) (Table 2
The analysis of geocoded goods and services showed that individuals who had low SES in high-SES neighborhoods had significantly more resources for health care (e.g., primary care physicians and health care clinics) near their homes than did their counterparts in low-SES neighborhoods (Table 3
We examined the interaction between individual and neighborhood SES on risk of mortality among a predominately non-Hispanic White study population, and found excess mortality among adults who had low SES and lived in high-SES neighborhoods. This finding suggests that these individuals do not benefit from the higher quality of resources and knowledge generally associated with higher SES neighborhoods. Although the specific mechanisms that underlay the excess mortality are unknown, we consider 2 plausible explanations that are consistent with a relative deprivation model or a relative standing model. These may act alone or in combination. First, adults of low SES in high-SES neighborhoods may experience relative deprivation because they have less disposable income for essential goods and services (e.g., food, health care, medications, and transportation) because of factors such as higher housing costs. Higher costs in high-SES neighborhoods might also create a need for adults of low SES to work longer hours and therefore have less time to maintain or adopt healthy behaviors. Furthermore, adults of low SES in high-SES neighborhoods may live farther from essential goods and services or be more removed from social services and other resources (e.g., free community clinics, subsidized food and housing) than are adults who have low SES in low-SES neighborhoods. This may be particularly problematic if they also have limited access to transportation. We found little support for this from our analysis of geocoded goods and services. Adults of low SES in high-SES neighborhoods had more primary care physicians and health care clinics near their homes, as well as fewer grocery stores and banks, and fewer convenience stores and alcohol outlets (the latter 2 have been linked with negative health outcomes).42,43 Although proximity assures neither access nor quality, and goods and services may be used outside of neighborhoods, our data suggest that proximity to essential goods and services do not explain our findings. A second explanation for the excess mortality among adults of low SES in high-SES neighborhoods may be their low relative standing in their communities. It has long been suggested that the discrepancy between an individuals social position relative to others in his or her community may influence risk of death.16,4446 Low social position may also be associated with fewer resources to cope with stressful life events, lack of social support, and low sense of control, which may result in real or perceived social isolation, discrimination, or other psychosocial stressors.18,4450 This explanation is consistent with the finding that a range of psychosocial factors may affect health, either indirectly by influencing health behaviors, or directly by influencing neuroendocrine or immune functioning.50 It is important to note that there may be unrecognized benefits that adults of low SES gain from living in higher SES neighborhoods. It is possible, for example, that adults, despite having higher mortality, may benefit from a higher quality of life in other unmeasured ways or that their children may benefit from amenities in such neighborhoods, such as safer and higher quality schools.51,52
Previous Studies
Strengths and Limitations Our study also has limitations. Our death match was based on California death records that miss residents who die outside of California. We assessed the completeness of our California death match by conducting a death match that used both California and national death records for the first survey (19791980, n = 1603 people). There were few differences: the California death match identified 260 deaths and the national death search identified 18 additional deaths but missed 13 deaths. The 18 California deaths identified by the national match but missed by the California match were fairly evenly distributed among the 9 individual and neighborhood SES groups (< 2 deaths missed in all groups, except 6 deaths missed among individuals who had moderate SES in moderate SES neighborhoods). We also obtained California hospital discharge records of participants from the last survey (19891990, n = 1719 people with 627 hospitalizations). We repeated the survival curves for the same 9 individual and neighborhood SES groups and found an almost identical pattern for hospitalizations as for mortality.53 There were also higher proportions of Hispanic participants who had low SES in low-SES neighborhoods (42% for women, 38% for men) compared with the other 8 individual and neighborhood SES groups (2%28%). Deaths may have been missed in this group because of reverse migration, matching errors, or other reasons, which could result in an underestimate of the death rate for women or men in that category. We do not believe this is the case, however, given that we found similar results when we restricted the analyses to White, non-Hispanic participants. Another potential limitation is that factors associated with self-selection into certain neighborhoods could account for the results and lead to erroneous conclusions of neighborhood effects. Our neighborhoods were based on geographically defined census tract, block group boundaries, or both, and considerable debate exists as to whether such boundaries represent neighborhoods as defined by their residents.54,55 Finally, we did not measure the length of time people were exposed to their neighborhood environments and whether they lived in the same type of neighborhood over time (e.g., at time of death or censoring). Although 78% of participants reported living in "their community" 5 or more years, this does not guarantee that they lived in their census-defined neighborhoods, or in similar neighborhoods, after the survey.
Implications
This study was funded as part of the National Institute of Environmental Health Sciences Initiative, in collaboration with 6 other National Institutes of Health (National Heart, Lung, and Blood Institute grant RO1 HL67731). We thank Ying-Chih Chuang, Craig Pollack, Soowon Kim, May Wang, and Lauren Cochran for their comments on an earlier draft; David Rogosa for biostatistical assistance; and Alana Koehler for assistance with manuscript preparation. Human Participant Protection All research was approved by the administrative panels on human subjects in medical research at Stanford University School of Medicine and conforms to the principles of the Declaration of Helsinki.
Peer Reviewed
Contributors Accepted for publication October 30, 2005.
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