Asian men also worked through mail-order agencies to find wives as they worked overseas in the 1800s. Key variables determining the relationship between migration and marriage were demographics, legal policies, cultural perceptions and technology. Imbalances between the number of available women and the number of men desiring partners created a demand for immigrant women. As a result of this imbalance, a new system of "picture brides" developed in predominantly male settlements. In the early 20th century, the institution of "picture brides" developed due to immigration restrictions. The Japanese-American Passport Agreement of 1907 allowed Japan to grant passports to the wives of immigrants to America. As immigration of unmarried Japanese women to America was effectively barred, the use of "picture brides" provided a mechanism for willing women to obtain a passport to America, while Japanese workers in America could gain a female helpmate of their own nationality.