http://www.ted.com/talks/richard_wilkinson.html

This is the latest Ted Talk to cross my viewscreen.  It's Richard Wilkinson, speaking about the differences between societies with wide vs narrow differences between the highest and lowest income groups.  The finding is intuitive, but the specific data that he pulls together, and the way he makes sense of it, is very interesting.  At the end of brings it all together with some science about stress.  According to him, the stressors that cause the greatest increase in cortisol are "social evaluative threats" to one's esteem or status.  In other words, "people are sensitive to being looked down on".  In societies where there is greater equality, there is less stress, hence explaining the increased longevity, health and peace that is seen in those societies.  Of course, the US rates only second to Singapore in his scaling of wealth disparity, with Japan and Sweden at the other end of the scale.  Anyway, it's worth seeing for yourself, if you have the 15 minutes.