Brent suggested the following:
Perhaps more to your purposes as an addendum, a direct quote from Wilkinson:
"Although economic growth remains important in poorer countries, across the richest 25 or 30 countries, there is no tendency
whatsoever for health to be better among the most affluent rather than the least affluent of these rich countries. The same
is also true of levels of violence, teenage pregnancy rates, literacy and maths scores among school children, and even obesity
rates. In poorer countries both inequality and economic growth are important to outcomes such as health, but rich countries have reaches a level of development beyond which further rises in material living standards do not help reduce health or social problems. While greater equality is important at all levels of economic development, the connection between life expectancy and Gross National Income per head weakens as countries get richer until, among the very riches countries, the connection disappears entirely."
Brent also sent me this link and this note on the topic.
Primarily I thought the Dolen link could be useful as an addendum because of your article's discussion of cross-country comparison respective of GDP per capita. The Dolen link:
o gave SPI vs. GDP per capita (at PPP) for a much wider basket of countries than the basket of countries shown in the figure 7 that you provided in your article (and so might be useful because your article didn't state a restriction to developed countries); and
o gave the line "we could say that for the poorer countries, GDP “explains” about two-thirds of differences in SPIs, but for the wealthier countries, only about one third." (and so might be useful because of your intended focus on developed countries, and further useful as a direct quote as opposed to your interpretation of a video).