It's worse than people realize too because there's actually mean reversion. Even if you bring in the highest quality immigrants based on strict standards, once those immigrants procreate, their offspring has a tendency toward mean reversion, meaning the offspring tend to have genetics that are closer to the mean of their race rather than exactly like their superior immigrant parents. There's studies done on immigrants that essentially show this. The second and third generation immigrants actually tend to end up worse off than the first generation immigrants when the first generation immigrants were put through strict immigration standards such that they were high quality to begin with.