Migration
Wage gaps between rich and poor nations are huge. A migrant from Guatemala to the US increases her income by a factor of 6 (PPP adjusted). From Kenya to the UK is a factor of 7; from Vietnam to Japan, a factor of 9.
A century ago, factors of 3 were sufficient to spark the migration that helped build America and partially empty Ireland, Italy and Poland.
Book review. No, not THAT book,” Ethan Zuckerman’s review of Lant Pritchett’s Let Their People Come