Great Replacement Theory

I've read that there is something called The Great Replacement Theory. I think it has to do with either a fear of a reduction in the percentage of the U.S. population that is White or a realization that the White percentage has fallen and almost certainly will continue to fall.

I think one reason that the White share of the population is falling is that many immigrants are dark-skinned.

I think that a second reason is that affluent people have fewer children. Why? I think poor people have more trouble affording children.