Name one nation in history that has improved due to mass immigration of people with different culture
There aren't really very many historical examples, as historically countries have been very isolationist.
However, it's only common sense that having more people will make your economy better. Nobody is going around without a single cent worth of wealth or income, and In fact the average american makes $55,000 annual income.
Edited 6/3/2016 19:22:08