Until the late 18th century, indentured servitude was common in British North America. It was often a way for Europeans (usually from Ireland) to immigrate to the American colonies: they signed an indenture in return for a costly passage. However, the system was also used to exploit Asians (mostly from India and China) who wanted to migrate to the New World. These Asian people were used mainly to construct roads and railway systems. After their indenture expired, the immigrants were free to work for themselves or another employer. It has been theorized by at least one economist that indentured servitude occurred largely as "an institutional response to a capital market imperfection".