https://www.brookings.edu/articles/disquiet-in-the-worlds-middle-class/
https://www.esri.ie/system/files/publications/RB202019_0.pdf
https://www.socialeurope.eu/wp-content/uploads/2018/12/SD_ProgressiveSociety-SustainableEquality.pdf
https://www.thelancet.com/action/showPdf?pii=S0140-6736%2820%2930461-X