https://www.esri.ie/system/files/publications/RS158_0.pdf
http://blogs.lse.ac.uk/brexit/2017/06/05/uk-voters-including-leavers-care-more-about-reducing-non-eu-than-eu-migration/
https://www.economiesuisse.ch/de/artikel/kuendigungsinitiative-ii-eine-wohlstandsbremse-par-excellence
https://www.un.org/esa/population/publications/migration/migration.htm