It would have been nice if there were some hints given here as to at least why.

Here’s the study’s limited comment about causality: “Colds and influenza as predicted by SES could be due to differential exposure to infection, with less educated or less affluent people having more exposure because of more crowded living and working conditions or greater use of public transportation, or it could be due to differential vulnerability to infection. Headache and pain as predicted by SES could be related to differences in noise exposure, working environments, and posture or to chronic mild neuroendocrine activation and increased muscle tone due to chronic stress.

A consideration for interpreting these results is potential reverse causality: low income could cause poor health outcomes or poor health could cause individuals to have low income. However, poor health is more likely to be a cause of low income for long-term, chronic conditions, like diabetes, than for transitory colds or influenza. Moreover, education is determined at comparatively young ages and is therefore less susceptible to arguments about reverse causality.”

A possible cause of the increased problems with illness may be due to stress as a person sees others doing well because of advanced education and therefore a feeling of sub-clinical depression that leads to less care about his own health. The differences would not be noticable in the individual but would show up in a population.

Perhaps what is being perceived as the effect is actually the cause. What about this: People who get sick more drop out of school more often.