Assume you are judging immigration policy from a nationalistic point of view for the receiving country. Furthermore, assume that you have in place, or could have in place, a mechanism for positive selection of migrants. That is, you can attract some real talent, some Edward Tellers and Piet Mondrians.
I then wonder if for any desired amount of immigration, whether it should not be taken from countries with equal or lower birth rates than the receiving country.
Let’s say you have country X — Ruritania — with a relatively high birth rate. In that country, the relatively talented people are having a fair number of children and producing a decent “rate of return” on their talent. Why pluck them from that multiplicative environment? A generation or two later, the prospects from that country will be all the brighter.
Conversely, if you take in talent from a country with an equal or lower birth rate to yours, you are not lowering the future rate of potentially talented babies born.
Note that if you wait to take in talent from Ruritania, it is not because you don’t want them. It is because you want more/better of them later in the future.
Furthermore, if you are taking migrants from Ruritania in the future, when their birth rates are declining, you are also taking in individuals who, in the Beckerian sense, are receiving relatively high quality human capital investments from their parents, at least relative to that country’s past.
The perceptive will note that this argument holds regardless of the absolute number of migrants you might wish to take in, high or low.