In the case of economic migration (usually labor migration), differentials in wage rates are common. If the value of wages in the new country surpasses the value of wages in one's native country, he or she may choose to migrate, as long as the costs are not too high. Particularly in the 19th century, economic expansion of the US increased immigrant flow, and nearly 15% of the population was foreign born, thus making up a significant amount of the labor force.