[kwhitefoot]

Most likely inequality has an effect in addition to any problems caused by the absolute level of income. Remember that this study was done in Denmark which has a functioning welfare state (no doubt imperfect of course), so not very many of the people in question would suffer severe physical privation.

It would be interesting to see a similar study in a country where being in the bottom quintile also meant being absolutely poor.

[zozbot234]

Is it a real effect, or a mere correlation between relative low income and other kinds of social marginalization and/or lack of social capital? It's not always easy to measure the latter at an individual or household level (hence why relative SES is often used as a correlate), but there's plenty of research highlighting the importance of social capital provision at a broader community scale.