Traditional economic theory predicts that capital mobility and international trade will push the world's national economies to one income level. As poorer nations race ahead, richer ones should slow down. Eventually, theory says, national economies would reach equilibrium. The reality of the last few decades, however, defies this notion; most of the poorest economies continue to lag far behind. For 50 years, foreign aid has been the main way the international community has promoted economic development. Yet it has not proven to be a silver bullet.