9,966 research outputs found
Trends in quality-adjusted skill premia in the United States, 1960-2000
This paper presents new evidence that increases in college enrollment lead to a decline in
the average quality of college graduates between 1960 and 2000, resulting in a decrease of 8
percentage points in the college premium. The standard demand and supply framework (Katz
and Murphy, 1992, Card and Lemieux, 2001) can qualitatively account for the trend in the
college and age premia over this period, but the quantitative adjustments that need to be made
to account for changes in quality are substantial. Furthermore, the standard interpretation of
the supply effect can be misleading if the quality of college workers is not controlled for. To
illustrate the importance of these adjustments, we reanalyze the problem studied in Card and
Lemieux (2001), who observe that the rise in the college premium in the 1980s occurred mainly
for young workers, and attribute this to the differential behavior of the supply of skill between
the young and the old. Our results show that changes in quality are as important as changes
in prices to explain the phenomenon they document
Estimating distributions of potential outcomes using local instrumental variables with an application to changes in college enrollment and wage inequality
This paper extends the method of local instrumental variables developed by Heckman and Vyt-
lacil (1999, 2001, 2005) to the estimation of not only means, but also distributions of potential
outcomes. The newly developed method is illustrated by applying it to changes in college enroll-
ment and wage inequality using data from the National Longitudinal Survey of Youth of 1979.
Increases in college enrollment cause changes in the distribution of ability among college and high
school graduates. This paper estimates a semiparametric selection model of schooling and wages to
show that, for fixed skill prices, a 14% increase in college participation (analogous to the increase
observed in the 1980s), reduces the college premium by 12% and increases the 90-10 percentile ratio
among college graduates by 2
Scalar field black holes
With a suitable decomposition of its energy-momentum tensor into pressureless
matter and a vacuum type term, we investigate the spherical gravitational
collapse of a minimally coupled, self-interacting scalar field, showing that it
collapses to a singularity. The formed blackhole has a mass (in
Planck units), where is the mass of the scalar field. If the latter has the
axion mass, eV, the former has a mass .Comment: 8 pages, no figure
Friedmann cosmology with decaying vacuum density
Among the several proposals to solve the incompatibility between the observed
small value of the cosmological constant and the huge value obtained by quantum
field theories, we can find the idea of a decaying vacuum energy density,
leading from high values at early times of universe evolution to the small
value observed nowadays. In this paper we consider a variation law for the
vacuum density recently proposed by Schutzhold on the basis of quantum field
estimations in the curved, expanding background, characterized by a vacuum
density proportional to the Hubble parameter. We show that, in the context of
an isotropic and homogeneous, spatially flat model, the corresponding solutions
retain the well established features of the standard cosmology, and, in
addition, are in accordance with the observed cosmological parameters. Our
scenario presents an initial phase dominated by radiation, followed by a dust
era long enough to permit structure formation, and by an epoch dominated by the
cosmological term, which tends asymptotically to a de Sitter universe. Taking
the matter density equals to half of the vacuum energy density, as suggested by
observation, we obtain a universe age given by Ht = 1.1, and a decelerating
parameter equals to -1/2.Comment: Accepted for publication in General Relativity and Gravitatio
Exact solutions of Brans-Dicke cosmology with decaying vacuum density
We investigate cosmological solutions of Brans-Dicke theory with both the
vacuum energy density and the gravitational constant decaying linearly with the
Hubble parameter. A particular class of them, with constant deceleration
factor, sheds light on the cosmological constant problems, leading to a
presently small vacuum term, and to a constant ratio between the vacuum and
matter energy densities. By fixing the only free parameter of these solutions,
we obtain cosmological parameters in accordance with observations of both the
relative matter density and the universe age. In addition, we have three other
solutions, with Brans-Dicke parameter w = -1 and negative cosmological term,
two of them with a future singularity of big-rip type. Although interesting
from the theoretical point of view, two of them are not in agreement with the
observed universe. The third one leads, in the limit of large times, to a
constant relative matter density, being also a possible solution to the cosmic
coincidence problem.Comment: Minor changes, references added. Version accepted for publication in
Classical and Quantum Gravit
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