Imagine a country with an average IQ of 100, some average amount of education (with some distribution), some average amount of capital per head (with some distribution of ownership of capital). Now add immigrants – 10% of the population – that are the same in every way. Same average IQ, same distribution of IQ, same average amount of capital and same distribution. They speak the same language. They have similar political traditions. In other words, it is as if the US had just peacefully annexed an imaginary country that’s a lot like Canada.
Would the original inhabitants gain economically from this merger? Strikes me that this could only happen from economies of scale – since nothing has changed other than a 10% increase in overall size. There might be some diseconomies of scale as well. I wouldn’t expect a big payoff. Except for Nawapa, of course.
Contrast this with a situation in which the extra 10% is fairly different – lower average IQ, much less education on average, don’t speak English. They don’t bring along a lot of capital. They have and bring along their native political traditions, like everyone, but theirs stink. I can easily see how those immigrants might have improved their economic lot but it’s kindof hard to see how bringing in people with low human capital benefits the original citizens more than bringing in people with considerably higher human capital. Yet it must, because adding more of the same clearly has a small effect, while adding in lower-skilled must have a big positive effect. Practically all the economists say so.