After adjusting for numerous health and lifestyle factors, we found trends of better cognitive function and less cognitive decline with both increasing education and increasing annual income level among community-dwelling health professionals aged 66 years or older. All these women had at least one to two years of education after high school.
There are several important strengths of this study. First, most previous investigations have focused on populations with substantially lower levels of education, while we were able to provide information on the effects of education and income at the higher end of the spectrum. Although we previously examined education and income in the Nurses’ Health Study, another population of health professionals, the range of educational attainment was somewhat narrower in that group, as the lowest educational level was a 3-year RN diploma, whereas this Women’s Health Study cohort also included women with 1 year of education after high school. In addition, income information in the current study is from individual household income data, whereas that of the Nurses’ Health Study was from the census tract median household income data and was thus likely less accurate. Finally, in this population of health professionals, several important confounding variables, such as healthcare access, are likely minimized; furthermore, the extensive information on a wide array of health and lifestyle variables allowed careful control for numerous potential confounding factors which may have important effects on the apparent relation between education/socioeconomic status and cognitive function.
There are some limitations to our study. It is possible that we underestimated effects of socioeconomic status if income alone insufficiently characterizes the full extent of subject’s economic situation; in particular, household income may imperfectly represent socioeconomic status if the women or their spouses were retired (subjects were generally in their sixties at trial enrollment). In this observational study, we could not fully control for confounding, and there may be unmeasured confounding factors, or residual confounding, which could partly influence our observed associations; this should be considered in interpreting our results.
Such uncontrolled confounding would likely result in overestimates of the true associations between education, income and cognition as women with higher education or income were generally healthier; nonetheless, since we observed relatively minor attenuation of our effect estimates after adjusting for numerous potential covariates
, it is unlikely that uncontrolled confounding could completely explain the strong relations we observed between education or income and cognition. Lastly, we administered a brief, telephone cognitive assessment, and thus cannot measure all aspects of cognition. We chose to focus our test battery on verbal memory because verbal memory is among the strongest predictors of eventual development of Alzheimer disease, and we thus address a major public health issue.
Similar to our findings, other large studies have generally reported beneficial effects of educational attainment on cognitive function among subjects with somewhat less education than ours. In cross-sectional studies of men, both Gallacher et al.[4
] in the Caerphilly Study and Cagney and Lauderdale[5
] found a significant trend of increasing cognitive scores with increasing education after adjustment for age and socioeconomic status. In a prospective study of 3,266 community-dwelling subjects over age 65 years, Evans et al. found that each level of education (ranging from 0 to 12+ years) predicted significantly less cognitive decline, after multivariate adjustment.[2
] Another study observed 70% lower risk of decline on the MMSE with increments in education (education categorized as <10 years, 10–12 years, or some college) in 14,833 subjects 18 years and older.[1
] A study with longer follow-up (11.5 years) of 1,488 men and women aged 18 years and older also found that more than eight years of formal education was associated with significantly less decline on the MMSE, after adjusting for age and race.[3
] In a recent study of 557 community-dwelling Spanish men and women over aged 65 years, less education (no primary school education vs primary school) was associated with increased odds of cognitive decline over four years (OR=2.36, 95% CI, 1.16–4.81).[7
] More recently, Seeman and colleagues, after following 895 community-dwelling men and women for three to seven years, observed increasingly better cognitive performance on tests of memory, naming, spatial recognition, figures/copying, and similarities with increasing educational attainment (categorized as 0–8 years, 9–11 years, 12 years, and college or more) increased.[9
] Finally, in the Nurses’ Health Study of 19,319 community-dwelling women aged 70–79 years, Lee and colleagues found a significantly decreased risk of cognitive impairment (OR=0.49, 95% CI, 0.36, 0.66) and of cognitive decline (RR=0.65, 95% CI, 0.50, 0.86) in women with a graduate degree compared with an RN diploma, after adjusting for potential confounders.[8
] Thus, the data reported here serve as important confirmation of the cognitive impact of education, even among well-educated subjects.
There are several possible mechanisms which can explain the association of education to cognitive function. First, education may simply reflect better health or other behavioral choices that in turn lead to maintenance of cognitive function.[15
] That is, the relation between education and cognition may be largely due to confounding.
However, our finding that effects of education persisted after carefully adjusting for numerous health and behavioral characteristics in a population of health professionals, relatively homogeneous in their health habits and access to healthcare, suggests that confounding by health status is not the most likely explanation
. Secondly, higher levels of education may improve an individual’s ability to perform on tests of cognitive function or to hide mild cognitive deficits.[15
] Again, in our group of educated professionals, it does not seem likely that this could completely explain the substantial differences in performance that we observed between women who are all highly educated.
Thus, our data suggest it is most likely that education exerts direct effects on brain structure or function
. One hypothesis proposes that education may act early in life by increasing synapse number or vascularization and creating cognitive reserves.[15
] This has been called the “reserve capacity” hypothesis, in which it is believed that those with a greater cognitive reserve can tolerate more brain disturbance before manifesting cognitive symptoms. Interestingly, a recent study indicated that level of education modulated the effects of cerebral white matter hyperintensities (WMH) on cognition;[16
] with significant associations between severe WMH and lower cognitive performances in participants with lower levels of education but not in those with higher education. Another study also suggested that the relation of neuritic and diffuse plaques to cognition was modified by level of education.[17
] An additional hypothesis is that education in early life may have effects in later life, if those with more education continue searching for mental stimulation (the “use it or lose it” hypothesis), possibly leading to beneficial neurochemical or structural alterations in the brain.[15
] Indeed, several studies have found that mental stimulation from social or leisure activities[19
] was associated with decreased incidence of cognitive decline and dementia.[21
Fewer research has addressed the effect of income on cognitive function, and results have not been entirely consistent. In their cross-sectional study, Cagney et al. reported that, compared with those whose yearly income was greater than $50,000, those with income below $5,000 had lower scores by 0.36 to 0.98 points (p<0.05) in tests of memory, working memory, knowledge, language, and orientation;[5
] net worth - consisting of value of home owned, checking or savings accounts, etc - had greater effects than income. In a prospective study, Evans et al. reported that each $5000 increase in income predicted a 0.03-point increase in immediate memory score (p=0.04), but they found no effect of income on the results of a mental status test.[2
] In a large-scale investigation of women with more than high school education, there was no effect of census tract median household income,[8
] although this is a relatively crude measure of income. Findings from our current study generally support a relation between household income and cognitive function. However, further study using alternate measures of socioeconomic status (eg, net worth) may be useful in better evaluating this relationship since household income alone may not fully represent socioeconomic status, especially in older persons. Moreover, the greater attenuation of effect estimates we found after multivariable adjustment in analyses of income than of education suggests that apparent relations with income may be partly mediated by health and lifestyle factors; thus, further research regarding mechanisms explaining the relation between income and cognition will be important.
Overall, we found clear relations between education, income, and cognitive function. In particular, this may point to feasible means for reducing cognitive decline if the relation with education might be explained by increased cognitive reserve or increased mental stimulation in those with higher levels of education.