@inbook{NBERc8909,
 Crossref = "NBERrose81-1",
 title = "Heterogeneity and State Dependence",
 author = "James J. Heckman",
 BookTitle = "Studies in Labor Markets",
 Publisher = "University of Chicago Press",
 pages = "91-140",
 year = "1981",
 URL = "http://www.nber.org/chapters/c8909",
}
@book{NBERrose81-1,
 title = "Studies in Labor Markets",
 author = "Sherwin Rosen",
 institution = "National Bureau of Economic Research",
 type = "Book",
 publisher = "University of Chicago Press",
 year = "1981",
 URL = "http://www.nber.org/books/rose81-1",
}