Elucidating the biological mechanisms linking socioeconomic position and health outcomes is an active area of research. This study looks at adult socioeconomic and cortisol in a diverse population-based sample of adults from Chicago across a wide age range, finding that lower educational levels were associated with a lowering of the overall diurnal cortisol profile. While most socioeconomic position and cortisol studies to date have relied on specialized samples that might make results difficult to generalize, the current study draws from a population-based sample. A major strength of this study is the use of non-parametric spline techniques to better identify the salient features of the diurnal pattern of secretion for this sample. An additional strength includes sampling on more than one day and the use of multi-level models to account for within and between person variation in cortisol patterns.
Limitations of this study include variation in collection times, which made one particularly time-sensitive feature of the diurnal cortisol pattern, the cortisol awakening response, difficult to detect. The time between waking and recording a “waking” cortisol sample has been shown to have important implications for the cortisol awakening response, with a delay of more than 10 minutes resulting in almost no cortisol awakening response on average (29
). New methods of compliance monitoring using microchips to record the opening of collection vials could help to minimize these problems, although these technologies are currently expensive for use in population based research (10
). While timing is especially important for the cortisol awakening response, over the rest of the day, variability in sample collection times was in fact advantageous for characterizing the diurnal pattern in more detail (23
). From the overall focal sample of 1145 individuals, the percentage of individuals both agreeing to participate in the saliva collection and returning usable samples was quite low (27%). While our sample was slightly older and less likely to be Hispanic than the overall sample, remaining measured sociodemographic and health characteristics did not differ. It is still possible that those who did not participate or successfully return usable samples were different in ways we did not observe, which may limit the generalizability of our findings. Given the measurement error associated with cortisol patterns, our sample size of 311 may also have limited our ability to detect a more finely graded association with education. Another limitation was the use of years of education as our single measure of socioeconomic position, something necessitated by the large fraction of respondents missing income data in our sample. While using years of education as a marker of socioeconomic position likely captures more permanent attributes of lifelong socioeconomic position, it is limited in its ability to reflect more current or transitory features of socioeconomic position that may contribute to the stressors and cortisol response of our sample. Our study is also cross-sectional in nature, so we cannot rule out the possibility that genetics or early life factors influence both educational attainment and adult cortisol response. Future studies that measure both socioeconomic position and cortisol throughout the life course can help shed light on these issues.
This paper’s findings contribute to the inconsistent findings in the literature concerning the association between socioeconomic position and the diurnal pattern of cortisol secretion (14
). To our knowledge, this study is the first to identify lower total free cortisol release among those with lower socioeconomic position as measured by area- under-the-curve. Previous work has found a higher area-under-the-curve among those with lower socioeconomic position or no association. In the CARDIA study, a higher area-under-the-curve was found as a result of similar morning levels but higher evening cortisol levels for those with lower socioeconomic position (19
), which appears to also be the case in the analysis of Li et al. with respect to morning and mid-day values (17
). In another study, Cohen et al. found non-significantly higher levels of cortisol with lower socioeconomic position for each of seven cortisol measurements throughout the day, attributing the area-under-the-curve difference to small difference accumulating over the day (20
). The Cohen study found no significant association between socioeconomic position and slope, suggesting a uniformly higher
pattern for those with lower socioeconomic position, in contrast to the uniformly lower
pattern found here. Recent results from the Multi Ethnic Study of Atherosclerosis found no association between income and wealth and area-under-the-curve, but those with the lowest levels of income and wealth had lower wake-up cortisol levels compared with the highest group, consistent with our results (22
Our findings of an overall lower diurnal cortisol pattern for those with less education are consistent with the findings of Brandtstadter, et al, who found that lower socioeconomic position was associated with lower levels of morning and afternoon cortisol in a sample of German adults, with no differences found in evening levels (16
). Steptoe, et al also reported a uniformly lower pattern for women with lower socioeconomic position in the Whitehall II studies, but the pattern was reversed for men (15
). Other studies have shown a blunted but higher diurnal pattern of cortisol secretion in those with lower socioeconomic position. Ranjit, et al found no differences in waking levels but flatter slopes over the course of the day for women reporting increased material hardship (21
Overall, existing discrepancies in the literature on socioeconomic position and salivary cortisol likely result from great variation in the collection and methods of analysis of salivary cortisol data. Intra-individual variation in patterns of cortisol secretion is known to be high, especially around time of awakening, making inter-individual comparisons based on a single day of collection likely to be noisy (13
). Of previous studies examining socioeconomic position and diurnal patterns of salivary cortisol secretion, only three collected measures for more than one day (20
), and only the recent Multi Ethnic Study of Atherslerosis study (22
) utilized a multi-level approach to separate out variability within or across individuals (13
These findings also highlight the lack of clarity in the literature on socioeconomic position and cortisol with respect to current research on cortisol and health outcomes. While earlier theoretical and animal models focused on chronic overexposure to cortisol as pathogenic, increasing evidence suggests that cortisol deviations in either direction are potentially harmful to health, and the importance of cortisol elevations or declines depends on the condition of interest (31
). The same can be said for the relationship between chronic stress and cortisol output, which for many years assumed that HPA activity always increased with stress. More recently, critical features of chronic stress that impact the direction of the HPA activity-stress relationship have emerged, including the time elapsed since stressor onset and the controllability of the stressor, among others(31
). While some work has begun investigating more specific elements of socioeconomic position and chronic stress with regards to cortisol such as job demands, job control, and relationship functioning (29
), overall efforts to link socioeconomic position to specific stressors and their implications for cortisol is lagging behind similar work in psychology and psychoneuroendocrinology. The net result of this lack of theoretical clarity has been to interpret any differences in the profile of cortisol secretion by socioeconomic position as supportive of the hypothesis that cortisol dysregulation is a link between low socioeconomic position and poor health outcomes, even if the direction of the socioeconomic differences in cortisol are found to be in the opposite direction in different studies.
In the current paper, the finding that lower socioeconomic position is associated with a uniformly lower
cortisol profile could potentially be explained in two ways: 1.) lower socioeconomic position is not associated with higher levels of chronic stress and subsequent increased HPA-activation as commonly proposed; 2.) lower socioeconomic position is associated with higher levels of chronic stress, but the long term chronic stress reflected in lower educational attainment results in a blunted diurnal rhythm and hypocortisolism, or chronic under-production of cortisol. The current study is consistent with both of these scenarios, and understanding which one better reflects reality will rely on continued progress in understanding the relationships between chronic stress and HPA-axis functioning (31
). While the current study has improved upon previous studies of socioeconomic position and cortisol associations that did not model the important within and between person differences in cortisol secretion, future investigations of socioeconomic position and cortisol patterns should test more specific hypotheses about how chronic and acute stressors associated with socioeconomic position impact HPA-axis activity.