@techreport{NBERw6106,
 title = "Income, Schooling, and Ability: Evidence from a New Sample of Identical Twins",
 author = "Orley Ashenfelter and Cecilia Rouse",
 institution = "National Bureau of Economic Research",
 type = "Working Paper",
 series = "Working Paper Series",
 number = "6106",
 year = "1997",
 month = "July",
 URL = "http://www.nber.org/papers/w6106",
 abstract = {We develop a model of optimal schooling investments and estimate it using new data on approximately 700 identical twins. We estimate an average return to schooling of 9 percent for identical twins, but estimated returns appear to be slightly higher for less able individuals.  Simple cross-section estimates are marginally upward biased.  These empirical results imply that more able individuals attain more schooling because they face lower marginal costs of schooling, not because of higher marginal benefits.},
}