"Economic historians examining trade and migration during the nineteenth century have offered compelling evidence that migration, both high- and low-skilled, provided higher gains than trade alone and have also suggested that migration may be a necessary condition to receive the overall economic gains of openness to trade and capital flows, due to total specialization or locational economics of sale (Hatton and Williamson 1998; O’Rourke and Williamson 1999). A neoliberal approach to global economic efficiency would call for unrestricted migration, allowing labor to move freely to the country where it earns the highest return and where its marginal product is the highest (Chang 1998)."