Many factors could disrupt the chinese miracle, the greatest economic expansion the world has ever witnessed. corruption and abuse of power afflict the ruling communist party, which must figure out how to placate a populace no longer satisfied simply by a full rice bowl. the nation is plagued by dirty air, foul water and a contaminated food chain. then there's the government's repression of independent voices, which human-rights activists say stifles creative impulses. yet perhaps the most dangerous drag on china,s future is a demographic one.