EDUCATION

Higher level of education on a given country leads to higher level of income

Higher level of education on a given country leads to higher level of income

Socio-economic status (SES) has long been a focus of interest for those seeking to elucidate the patterning of health disparities and the underlying causes for these disparities (Adler, Boyce, Chesney, Folkman, & Syme, 1993; Pincus & Callahan, 1995; Syme & Berkman, 1976). Decades of research have clearly delineated the fact that SES (as indexed by education, income and/or occupation) is a consistent and strong predictor of health differentials, with lower SES generally associated with poorer outcomes for most major types of morbidity and mortality (Adler et al., 1993; Crimmins & Cambois, 2003; Link & Phelan, 1995; Pincus & Callahan, 1995; Preston & Taubman, 1994).

The data also show that these health disparities reflect a gradient of effects, rather than a threshold effect of poverty vs. non-poverty, with those in the middle class experiencing better health than those below them but worse health than those above them (Adler et al., 1994; Marmot, 1994; Marmot et al., 1991; Pamuk, Makue, Heck, Reuben, & Lochner, 1998). Existing SES health gradients, however, remain incompletely understood.

Materials and methods

The study population reflects adults aged 20 and older (n=18,825) in the NHANES III (1988-1994), a nationally representative sample of the US population with interview, clinical exam, and laboratory components (National Center for Health Statistics, 1994). We excluded those with no mobile examination center visit (n=2252) (i.e., missing most biological data) and those missing education information (n=115) (our primary index of SES).

The household poverty income ratio, our second index of SES, was not used as an initial exclusion criteria due to greater missing data for this measure (an additional n=1413); rather, analyses based on the poverty income ratio were run on the subset of our general analysis sample who also had poverty income ratio data. Additional exclusion criteria included pregnancy (n=285) or excessive missing biological data (n=595; see below for details). The final analysis sample included a total of 15,578 NHANES participants.

Socio-economic status

Education was measured in terms of years completed as well as highest level completed (i.e., grade school, some high school, complete high school, some college, complete college or more). Indicator variables were created using “completed college or more” as the reference group to allow for examination of non-linear relationships to allostatic load. Income was measured based on the poverty income ratio, an index reflecting the ratio of household income to the household poverty level determined by area of residence and household size (Centers for Disease Control and Prevention (CDC), 1996).

Poverty income ratio values were examined both as a continuous measures and based on five categories (<1, 1-1.99, 2-2.99, 3-3.99, 4-4.99 and 5+) with those reporting incomes five (or more) times the poverty ratio serving as the reference group.

Ethnicity

Blacks and Mexican Americans were over-sampled, allowing for separate analysis of these two ethnic groups, along with whites and a fourth “other” category. Hispanics other than Mexican Americans were classified as ...