Background Although the relation between low income and poorhealth is well established, most previous research has measuredincome at only one time.
Methods We used income information collected in 1965, 1974,and 1983 from a representative sample of adults in Alameda County,California, to examine the cumulative effect of economic hardship(defined as a total household income of less than 200 percentof the federal poverty level) on participants who were alivein 1994.
Results Because of missing information, analyses were basedon between 1081 and 1124 participants (median age, 65 yearsin 1994). After adjustment for age and sex, there were significantgraded associations between the number of times income was lessthan 200 percent of the poverty level (range, 0 to 3) and allmeasures of functioning examined except social isolation. Ascompared with subjects without economic hardship, those witheconomic hardship in 1965, 1974, and 1983 were much more likelyto have difficulties with independent activities of daily living(such as cooking, shopping, and managing money) (odds ratio,3.38; 95 percent confidence interval, 1.49 to 7.64), activitiesof daily living (such as walking, eating, dressing, and usingthe toilet) (odds ratio, 3.79; 95 percent confidence interval,1.32 to 9.81), and clinical depression (odds ratio, 3.24; 95percent confidence interval, 1.32 to 7.89) in 1994. We foundlittle evidence of reverse causation — that is, that episodesof illness might have caused subsequent economic hardship.
Conclusions Sustained economic hardship leads to poorer physical,psychological, and cognitive functioning.
The relation between low income and poor health is well established.1,2,3,4,5Groups whose incomes are low are disproportionately exposedto social and psychological conditions that may have negativeeffects, while also possessing fewer economic resources to managethese circumstances.6 Low income may affect health directlythrough inadequate housing and sanitation or indirectly throughthreatening, socially disrupted neighborhoods and the promotionof behavior and psychosocial characteristics that are deleteriousto health.7,8,9
Most previous research has measured income at only one time.This method may fail to capture the health effects of sustainedexposure to low income or to account for transitions into andout of low-income groups. There is considerable volatility inincome over a lifetime, with 26 to 39 percent of people 45 to65 years of age having income reductions of 50 percent or moreat least once in an 11-year period.10 These rises and fallsin income are more pronounced for those at the bottom of theincome distribution, who are less likely to have stable employment.11Failure to account for such income dynamics could result inan underestimate of the true association between income andhealth status.12
We used income information collected in 1965, 1974, and 1983as part of the Alameda County Study to examine the cumulativeeffect of sustained economic hardship among members of the cohortwho were alive in 1994. The outcome measures represented importantfunctional aspects of day-to-day living, including physical,psychological, social, and cognitive functioning.
Methods
Study Population
We used information collected from the Alameda County Study,a population-based prospective investigation of predictors ofhealth and functioning in a representative sample of adultsin Alameda County, California. Full details of the sample in1965 (6982 respondents) and 1974 (4864 respondents) have beenpublished previously.13 In 1983, a random sample of 50 percentof the 1974 respondents who were not known to be dead in 1982were enrolled in a third period of data collection (a totalof 1799 respondents). In 1994, a fourth period of data collectionwas conducted for the 1974 respondents. Of the 3161 subjectswho were known to be alive, 3005 were located; 2935 were ableto participate, and 205 refused to participate, leaving a sampleof 2730. Response rates for all periods were high, with 86,85, 87, and 93 percent responding in 1965, 1974, 1983, and 1994,respectively. The results presented here are based on informationon income, risk factors, and prevalent diseases gathered in1965, 1974, and 1983 and levels of physical, psychological,cognitive, and social functioning in 1994. All analyses arerestricted to the 1799 respondents who were part of the random50 percent sample enrolled in 1983. Of these 1799 subjects,362 had died and 151 were lost to follow-up between 1983 and1994, leaving a possible sample of 1286. Analyses excluded subjectswho had missing data on income (130 respondents) or covariates(29) in 1965, 1974, or 1983. In addition, between 3 and 43 respondentshad missing information on measures of function, leaving a totalsample of between 1081 and 1124, depending on the functionaloutcome. There were no significant differences in demographic,health, or other risk factors between subjects with missingdata at base line and those without missing data (data not shown).The maximal sample included 325 women and 263 men who were 45to 64 years of age in 1994, 273 women and 224 men who were 65to 84 years old, and 29 women and 10 men who were 85 or olderin 1994.
Assessment of Sustained Economic Hardship
We calculated the number of times between 1965 and 1983 thatsubjects reported total household incomes that were less than200 percent of the federal poverty level for that year (thestudy definition of economic hardship); the results ranged from0 (never below 200 percent) to 3 (below 200 percent in 1965,1974, and 1983). The respective income cutoffs for 200 percentof the poverty level in 1965, 1974, and 1983 were $6,634, $11,000,and $20,356 for a four-person household.14 Income was self-reported,pretax household income from all sources at each period of datacollection between 1965 and 1983, recorded in income categories.The category midpoint was adjusted for family size and usedto identify households with incomes that were less than 200percent of the poverty level, based on U.S. Census informationfor the relevant year.14
Assessment of Functional Outcomes
Physical functioning was assessed with three commonly used measuresof functional status: activities of daily living,15 instrumentalactivities of daily living,16 and a physical performance scaledeveloped by Nagi.17 These scales are based on Likert-type self-reportsof the difficulty involved in walking, bathing, brushing hairor teeth, eating, dressing, moving from bed to chair, and usingthe toilet (activities of daily living); cooking, shopping,doing housework, using the telephone, and managing money (independentactivities of daily living); and pushing or pulling, lifting,getting up from a crouch or bend, reaching, stooping, standingup, walking up stairs, and handling small objects (Nagi performancescale). Subjects were classified as having reduced physicalfunctioning in 1994 according to the activities-of-daily-livingscale (82 of 1124 subjects, or 7.3 percent) and the independent-activities-of-daily-livingscale (164 of 1116 subjects with data available, or 14.7 percent)if they reported difficulties with two or more items, and accordingto the Nagi scale (130 of 1108 subjects with data available,or 11.7 percent) if they reported difficulties with five ormore items.
Psychological functioning was assessed in terms of depression,cynical hostility, and optimism. Depression was measured onthe basis of scores on 12 items that used the diagnostic symptomcriteria for a major depressive episode outlined in the Diagnosticand Statistical Manual of Mental Disorders, third edition, revised(DSM-III-R).18 Eighty-seven of 1120 subjects with data available(7.8 percent) with the highest scores were classified as clinicallydepressed. Depressive symptoms were measured with a scale developedin the Alameda County Study that has predictive validity forvarious outcomes.19 Subjects with scores of 5 or more on the18-item scale were classified as having symptoms of depression;140 of 1121 subjects (12.5 percent) met this criterion. Cynicalhostility was measured with a seven-item subgroup of the Cook–Medleyhostility scale that is predictive of carotid atherosclerosis.20Subjects with scores of more than 18 on the 28-point scale (foundin 23.4 percent of the subjects) were classified as having cynicalhostility. Optimism was measured with the Life Orientation Test.21A total of 245 of 1090 subjects (22.5 percent) had scores lowerthan 17 on a 24-point scale and were categorized as lackingoptimism about the future. Both the cynical-hostility scaleand the Life Orientation Test have been used in clinical andepidemiologic studies and are internally consistent, reliable,and predictive of health outcomes in a variety of populations.20,22
Cognitive function was assessed by four Likert-type questionsabout self-reported difficulties in remembering things, payingattention, finding the right word, and forgetting where thingswere placed. Those with difficulties with two or more of thesetasks were classified as having cognitive difficulties; 128of 1116 subjects (11.5 percent) met the criteria. The degreeof social isolation was based on the number of contacts withfriends or close friends subjects had each month; 204 of 1108subjects (18.4 percent) with data available who had fewer thantwo contacts a month were classified as being socially isolated.23
Assessment of Covariates
Smoking history, consumption of beer, wine, and liquor, physicalactivity, and body-mass index (the weight in kilograms dividedby the square of the height in meters) were assessed by questionnairein 1965, 1974, and 1983.24 Among all subjects, smoking was calculatedin terms of pack-years of exposure. Physical activity was measuredaccording to the frequency of activities such as walking, swimming,participation in exercise programs and vigorous sports, gardening,and fishing. To account for changes in body-mass index, physicalactivity, and alcohol consumption over time, we measured thenumber of periods in which participants were in the group withthe highest risk. For instance, we divided body-mass index intoquartiles for each period and counted the number of periods(0, 1, 2, or 3) in which the subject was in the highest quartilebetween 1965 and 1983.
Prevalent diseases in 1965, 1974, and 1983 were assessed byself-report; the diseases assessed have been demonstrated inthis population to be strongly associated with an increasedrisk of death.25 At each period, participants were asked whetherthey either currently had or had ever had heart trouble, chestpain, hypertension, a stroke, breathing difficulties, chronicbronchitis, diabetes, asthma, arthritis, back pain, or cancer.For each subject we counted the number of periods (0, 1, 2,or 3) in which each condition was present and included it asa continuous variable in all analyses.
Statistical Analysis
We assessed associations between economic hardship and functioningwith multivariate logistic regression,26 using the Proc Logisticprocedure in SAS version 6.1227 on a Sun workstation. Associationswere examined in models that were adjusted for both age andsex and in models that were adjusted separately for age, sex,and risk factors and for age, sex, and prevalent diseases. Weassessed the association between mortality and economic hardshipwith time-dependent proportional-hazards regression using thePHREG procedure in SAS version 6.12.28
Results
Table 1 shows the demographic, behavioral, and health characteristicsof the subjects according to the duration of economic hardship(income less than 200 percent of the poverty level). Table 2presents the results of logistic-regression analyses of theassociation between the duration of economic hardship and functioning.In models adjusted for age and sex, there were strong, significant,graded associations between the number of periods of economichardship and all measures of functioning except social isolation.We found no systematic differences in the pattern of associationsbetween sustained economic hardship and functioning betweenwomen and men or between subjects who were 65 years of age orolder and those who were under 65 (data not shown).
Table 2. Odds Ratios for Reduced Levels of Physical, Psychological, Cognitive, and Social Functioning in 1994 According to Whether There Was Sustained Economic Hardship between 1965 and 1983.
For physical functioning, subjects whose incomes were less than200 percent of the poverty level in 1965, 1974, and 1983 had3.79 times (95 percent confidence interval, 1.32 to 9.81) theodds of having difficulties with activities of daily livingin 1994 of those who had no history of economic hardship. Additionaladjustment for the number of pack-years of smoking, body-massindex, levels of alcohol consumption, and physical activityreduced the odds ratio to 2.86 (95 percent confidence interval,1.00 to 8.24). Adjustment for age, sex, and prevalent diseasesresulted in an odds ratio of 2.95 (95 percent confidence interval,1.02 to 8.58). Similar results were obtained when associationsbetween sustained economic hardship and other functional outcomes(e.g., independent activities of daily living and the Nagi performancescale) were adjusted for these risk factors.
There were equally strong, graded relations between sustainedeconomic hardship and measures of psychological and cognitivefunctioning. For instance, as compared with subjects with nohistory of economic hardship, subjects with three episodes ofeconomic hardship between 1965 and 1983 had 3.24 times (95 percentconfidence interval, 1.32 to 7.89) the sex- and age-adjustedodds of meeting the DSM-III-R criteria for depression. Similarly,the group with the longest history of economic hardship hadgreater odds of having high levels of depressive symptoms (oddsratio, 4.56; 95 percent confidence interval, 2.07 to 10.07),being cynically hostile (odds ratio, 5.09; 95 percent confidenceinterval, 2.40 to 10.86), lacking optimism (odds ratio, 5.68;95 percent confidence interval, 2.73 to 11.83), and having greaterself-reported difficulties with cognitive functioning (oddsratio, 4.60; 95 percent confidence interval, 2.06 to 10.32)in 1994.
Table 3 shows complete models for the associations between reducedactivities of daily living and economic hardship adjusted separatelyfor risk factors and prevalent diseases. In addition to sustainedeconomic hardship, smoking, body-mass index, and histories ofdiabetes, arthritis, and back pain were all significantly associatedwith the outcome.
Table 3. Adjusted Odds Ratios for Reduced Activities of Daily Living in 1994 According to the Duration of Economic Hardship between 1965 and 1983.
Reverse Causation
We examined reverse causation — the possibility that poorhealth and functioning caused economic hardship and not viceversa — in three ways. First, we examined associationsbetween sustained economic hardship and functioning in a samplerestricted to 982 subjects who were less than 50 years of agein 1965 and who had no reduction in physical functioning. Inthis subgroup any association between sustained economic hardshipand subsequent poor functioning could not be due to illnessat base line, because the prevalence of physical and cognitivedysfunction was extremely low in this age group. The magnitudeand pattern of associations with functioning were virtuallyidentical to those in the whole sample (data not shown).
Second, we restricted the sample to 2307 subjects who reportedexcellent or good health in 1965 and examined the effects ofeconomic hardship in 1965 on levels of functioning in 1994,after adjustment for pack-years of smoking, body-mass index,physical activity, alcohol consumption, and prevalent diseasesin 1965. Economic hardship in 1965 was a significant predictorof reduced physical, psychological, and cognitive functioningin 1994. Although these results mirrored the main findings,the associations were expectedly weaker. Odds ratios for reducedfunctioning ranged from 1.36 (95 percent confidence interval,0.99 to 1.85) for depressive symptoms to 1.64 (95 percent confidenceinterval, 1.19 to 2.27) for cognitive difficulties.
Third, we restricted the sample to 197 subjects whose incomewas not derived from wages or salaries in 1965 or 1974 and whowere in excellent or good health in 1965. Even in this smallsample, sustained economic hardship had significant and, inmany cases, graded associations with functional outcomes thatwere very similar to the patterns observed in the whole sample(Table 4). Even among subjects whose income did not depend onwages, those with economic hardship in 1965, 1974, and 1983had an odds ratio of 5.91 (95 percent confidence interval, 1.38to 25.29) for difficulties on the Nagi performance scale (Table 4).
Table 4. Odds of Reduced Levels of Physical, Psychological, Cognitive, and Social Functioning in 1994, According to Whether There Was Sustained Economic Hardship between 1965 and 1983, among Subjects Whose Income Was Not Derived from Wages or Salaries in 1965 and 1974 and Who Reported Excellent or Good Health in 1965.
Mortality
The associations reported here were evident even though subjectswith incomes below 200 percent of the poverty level were morelikely than those without economic hardship to have died duringthe study and therefore not to have been included in the analyses.Selective analysis of mortality with time-dependent covariateproportional-hazards regression, which allowed economic hardshipto vary as a function of the length of survival, showed thatsubjects whose incomes were less than 200 percent of the povertylevel were at increased risk for death over the 29-year studyperiod, but this risk differed according to age. As comparedwith subjects of the same age without economic hardship in 1965,those who were 35 years of age with economic hardship had anage- and sex-adjusted relative risk of death of 1.70 (95 percentconfidence interval, 1.29 to 2.22), and those who were 65 yearsof age with economic hardship had a relative risk of death of1.29 (95 percent confidence interval, 1.16 to 1.43).
Discussion
Our results demonstrate strong, consistent, graded associationsbetween sustained economic hardship from 1965 to 1983 and reducedphysical, psychological, and cognitive functioning in 1994.The associations were not greatly attenuated after adjustmentfor risk factors or prevalent diseases, even though these covariateswere related to many of the functional outcomes.
This study has several strengths. First, the findings were basedon multiple measures of income over a 17-year period and showdose–response associations between the number of periodsof economic hardship and important functional measures. Second,the graded associations were consistent across various measuresof functional status, but there was no relation between sustainedeconomic hardship and social isolation. This result is perhapssurprising, given other evidence that has generally shown thatlower-income groups have less social support.29,30 However,the associations may depend on the aspect of support that ismeasured and the reasons for the initiation of social contact.13The absence of an association between economic hardship andsocial functioning may have been because reduced functioningand higher levels of psychological distress translated intoan increased need for social contacts to deliver help. Third,in additional analyses, these results were not sensitive tochanges in the cutoff points that defined the functional outcomes(data not shown).
Although the prospective, dose–response associations weobserved may suggest a causal relation between sustained economichardship and poor functioning, it was important to explore thepossibility of reverse causation (i.e., that episodes of illnesscaused subsequent economic hardship and not vice versa). Weaddressed this issue in three ways. We examined associationsbetween periods of economic hardship and functioning in a subgroupthat was healthy at base line and found almost identical relationsbetween more sustained economic hardship and reduced functioning.However, although this analysis showed that illness at baseline could not account for the associations, it did not precludethe possibility that illness after base line caused subsequenteconomic hardship. We conducted another analysis examining onlythe effects of economic hardship in 1965 on functioning in 1994in a subgroup that was healthy at base line. Any associationbetween an income that was less than 200 percent of the povertylevel in 1965 and functioning in 1994 would not rely on repeatedepisodes of low income that could have been the consequenceof illness after base line. In this subgroup, one period ofeconomic hardship in 1965 was a significant predictor of reducedphysical, psychological, and cognitive functioning in 1994.
In a more stringent examination of the plausibility of reversecausation as an explanation for the findings, we again restrictedthe sample to subjects who reported excellent or good healthin 1965 and whose income was not derived from wages or salaries.In this sample, episodes of illness were less likely to createeconomic hardship because the sources and level of income (e.g.,partner's income, investments, welfare, and pensions) did notdepend directly on the subject's health status. Sustained economichardship had significant and in many cases graded associationswith functional outcomes that were very similar to the patternsobserved in the whole sample. Under certain conditions, episodesof illness may affect the ability to generate income,31 butgiven the results of these analyses of subgroups we found verylittle evidence that reverse causation could explain the overallmagnitude and pattern of the findings.
The associations were evident even though subjects with moresustained economic hardship between 1965 and 1983 were morelikely to have died before the 1994 survey and to have beenexcluded from these analyses. This increased risk of death shouldhave reduced the likelihood of finding associations betweensustained economic hardship and functioning in those who surviveduntil 1994.
Despite the fact that the measures of risk factors and prevalentdiseases used in this study were related to the functional outcomesin multivariate models, they failed to attenuate the associationsbetween sustained economic hardship and functioning. Cautionshould be exercised in interpreting this absence of confoundingas evidence that the association between sustained economichardship and functional status is largely independent of thesefactors, since it is reasonable to think of them as potentiallyintervening sources.1,32 It is possible that the measures usedhere did not fully capture the cumulative effects of these riskfactors and prevalent diseases. Furthermore, without informationon the extent of measurement error, the degree and directionof potential bias involved in estimating residual multivariateassociations cannot be predicted.33
Our results show the cumulative health effects of sustainedeconomic hardship and have potentially important implicationsfor public health, health care, and economic policy. Peoplewith sustained economic hardship are more likely to have poorerphysical, psychological, and cognitive functioning that mightbenefit from medical intervention, but recent evidence showsthey are less likely to receive such care.34,35 It will be importantto monitor these trends, especially in the light of the uncertaintiessurrounding welfare reform and transitions to managed care.36Furthermore, increases in income inequality in the United States37suggest that larger proportions of the population, especiallychildren, have been pushed into low-income groups, and economicpolicies that polarize the income distribution may have seriousshort-term and long-term health consequences.38,39 In conclusion,sustained economic hardship leaves physical, psychological,and cognitive imprints that decrease the quality of day-to-daylife.
Supported by a grant (1R37AG11375) from the National Instituteon Aging.
Source Information
From the Department of Epidemiology, School of Public Health, University of Michigan, Ann Arbor (J.W.L. G.A.K.), and the Human Population Laboratory, Public Health Institute, Berkeley, Calif. (S.J.S.).
Address reprint requests to Dr. Lynch at the Department of Epidemiology, School of Public Health, University of Michigan, 109 Observatory St., Ann Arbor, MI 48109-2029.
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