You are suggesting that the U.S middle class is suffering more due to the undemocratic nature of current U.S government? Interesting thought, a bit hard to prove in the current economic climate because all the middle classes of all the 1st world countries are suffering.
I was actually thinking more along the line of average life-expectancy-at-birth statistics compared by top economic quintile (20%) for many different nations. Life expectancy at birth is always lowest for the lowest quintile income group. Other people have demonstrated a positive correlation with low life-expectancy-at-birth numbers with income inequality for comparable national GDP's The problem is comparing GDP's as well as how "democratic" a nation is.
People immediately bring up different national data reporting requirements, as well as errors in collection. However, thing have improved over the years, and both people and governments find this type of data worth acting on. Perfection is not required. I seem to remember that Sweden's average life-expectancy-at-birth statistics are quite good. Maybe it is just the cold air.
As a democracy activist I find this an interesting political issue.