If an additional infant born in America has positive externalities, asks Adam Ozimek, shouldn’t an additional immigrant who moves to America have positive externalities, too?

Since the average immigrant age is around 30, that means when they’ve
arrived they are already past the stage when they just consume
society’s resources, and have begin producing externalities. Doesn’t
this suggest that positive externality of immigrants is even larger
than that for natives? If you’re going to claim that the average lower
education level of immigrants reduces their externalities, keep in mind
that immigrants are also more likely to be small business owners and
PhDs than natives.

Another objection is that immigration creates positive externalities for the country of destination, but negative externalities for the country of origin.  But when you remember remittances, the objection falls flat.  Workers do a lot more for the Third World by leaving than staying.