@inbook{NBERc6664,
 Crossref = "NBERabow91-1",
 title = "Undocumented Mexican-born Workers in the United States: How Many, How Permanent?",
 author = "George J. Borjas and Richard B. Freeman and Kevin Lang",
 BookTitle = "Immigration, Trade and the Labor Market",
 Publisher = "University of Chicago Press",
 pages = "77-100",
 year = "1991",
 month = "January",
 URL = "http://www.nber.org/chapters/c6664",
}
@book{NBERabow91-1,
 title = "Immigration, Trade and the Labor Market",
 author = "John M. Abowd and Richard B. Freeman",
 institution = "National Bureau of Economic Research",
 type = "Book",
 publisher = "University of Chicago Press",
 year = "1991",
 URL = "http://www.nber.org/books/abow91-1",
}