As an example, my translator from Afghanistan received a special immigrant visa to immigrate here, for his service. He brought his wife and son.
They had two more kids, and had their citizenship paperwork ready as soon as five years passed. Within a year of citizenship, they are sponsoring the wife’s parents, who though old and sick, have five children under 18 who would come for the ride. That will make 12 new Americans in 6 years. His side of the family isn’t coming over just yet, but there are a lot of them.
One immigrant can literally grow into a dozen, or even several dozen in under a decade through chain migration.
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