While the U.S., India, Russia and Japan may maneuver to limit China's expanded reach, there are several domestic liabilities that could potentially limit Beijing's ability to gain its presumed position in the region. Imbalance has caused one of the largest migrations in the world's history as peasants from China's western provinces relocate to the booming economies of Shanghai, Beijing and Guangzhou. More than 40 percent of China's population now live in cities or towns, up from 18 percent in 1978. Environmental damage may not be the only legacy of China's centrally planned economy. China's "one child" policy may have created a society with far fewer workers than necessary to care for a population that will be dramatically weighted toward the elderly in the coming decades. Under the "one child" policy, male children were favored over females. Soon there will be an abundance of young men with no prospects for marriage. This could prove to be a destabilizing factor. China has recently begun to emerge as a great power, but much of this power is derived from the perception that its rapidly expanding economy will continue to raise the boats of its neighbors. Should China's economy begin to sputter, this newfound power could rapidly dry up.