The book Among the Hidden by Margret Peterson Haddix?

I don't think writers generally write popular fiction for a "purpose" other than to entertain and make money and have fun with the writing process. I do think, though, that the book deals with issues of over-population and probably has to do with the author's views on countries like China that have childbearing laws to prevent over-population. That's an umption, but I think it's a probably a pretty accurate one.