The Census Bureau has been collecting and tabulating data on residential mobility for almost 150 years, beginning with questions about state of birth included in the 1850 census. The 1940 census introduced a fixed-period migration question (where a person lived five years ago) and obtained information on the origin, destination, and a wealth of social and economic characteristics. As a result, the existing database on geographical mobility is very rich in terms of both historical perspective and characteristics of migrants.