Income, Stress and Blood Pressure

Epidemiologists study how health and disease are distributed in populations and which factors explain the patterns. Researchers have found that people with lower income and less education tend to have worse health. This relationship is often described as a gradient, because health improves at each step up the social ladder and not only at the extremes. Possible explanations include differences in health behaviors such as smoking, in access to healthy food and health care, and in exposure to chronic stress. Researchers must also consider reverse causation, in which poor health lowers income, and confounding by other variables.

Part 1. A research team surveys 8,000 adults in one metropolitan area, selected from a list of patients registered at community health clinics. Each participant has a blood pressure measurement taken during the visit, and is classified as having hypertension if the average of two readings is above a standard cutoff or if they are taking medication for high blood pressure. The participants are divided into four equal groups by household income (quartiles). Table 1 shows the percentage of each group with hypertension.

Table 1. Prevalence of hypertension by household income quartile.

Prevalence is the proportion of people who have a condition at a given time, while incidence is the proportion who develop it during a period. Because Part 1 counts people who already have hypertension, it measures prevalence. When a statistical model is adjusted for a variable, it estimates the association between income and hypertension among people who are similar on that variable.

Part 2. The researchers then compare the lowest and highest income groups using relative risk, the ratio of the prevalence in one group to the prevalence in the other. They calculate the relative risk before and after adjusting for other variables in a statistical model. Table 2 shows the results.

Table 2. Relative risk of hypertension in the lowest compared with the highest income quartile.

Part 3. In a separate survey of 800 adults, the researchers ask participants to complete a questionnaire about how often they experience unfair treatment in daily life, which produces a score. They also record each participant’s systolic blood pressure. The correlation between the score and systolic blood pressure is r = 0.30.

The researchers note that income was measured only once, at the time of the survey, and that participants who did not have a recorded income were not included in the analysis. They also point out that the clinics serve communities with a wide range of incomes, but that people who have no regular access to a clinic could not be included.