Immigration is also literal enstupidation as well as war, as per Israeli military historian Martin van Creveld. The great irony, of course, is that immigration is also, contra its dishonest defenders, a net negative for the economy.
Imperialism boosts an economy, both overall and per capita. Immigration causes it to grow overall in terms of GDP, but to decline on a per capita basis. Immigration is one of the two reasons that wages have fallen since 1973 and has contributed to the inability of the GDP growth rate to return to the level of the 1950s and 1960s.