332
May 2001
Amer. J. Agr. Econ.
after when observed out of school, t21 iswork at age 21 or the earliest year thereafter when
experience at thistime, and the g are, aspre-
viously defined, the growth rates of earnings
upon migration and remaining in the local
non-metropolitan area.2
observed out of school were not available for
420 of these individuals. County of residence
wasunavailable for three additional individ-
uals, leaving a study sample of 1476 per-
sons. An individual is considered a migrant
if, by age 21 or the earliest year thereafter
when observed out of school, he or she is
found residing in a county other than the
non-metropolitan county of residence at 14.
Migrantsto metropolitan areasand to other
non-metropolitan areascomprise 30.8% and
15.7%, respectively, of the total sample.3
Supplemental data on county unemploy-
ment ratesfor the years1975 and 1979 were
obtained from the Bureau of Labor Statis-
tics. Information on 1975 and 1980 county per
capita income, 1980 population per square
mile, and 1980 agricultural and manufac-
turing employment shares were obtained
from the 1983 County and City Data Book,
U.S. Department of Commerce, Bureau
The log of worker i’sobserved hourly earn-
ing isln Wiꢀ 21ꢀ M = Xi1B1 + gU t21 + ei1 if I ∗ =
Ziꢀ + vi > 0 and ln Wiꢀ 21ꢀ N = Xi2B2 + gRt21 +
ei2 if I∗ = Ziꢀ + vi ≤ 0. Since cov(ei1ꢀ vi)
ꢂ =0
and cov(ei2ꢀ vi)
ꢂ =0, estimating (8.1) and (9.1)
by OLS applied, separately, to samples of
migrantsand non-migrantsmay yield incon-
sistent estimates for the full sample. Assum-
ing ei1 and vi, and ei2 and vi, are bivariate
normally distributed, (8.1) and (12), and then
(9.1) and (12), may be consistently estimated
by the maximum-likelihood method of type 2
tobit (Amemiya).
With consistent estimates of B1 and B2,
ꢅ
ꢅ
namely B1 and B2, worker i’sexpected ini-
tial hourly earningswith no work experience
ꢀ
upon migration ln Wiꢀ 0ꢀ M and upon remain-
ꢀ
ing in the county of origin ln Wiꢀ 0ꢀ N may be of the Census. County-level employment-
ꢅ
ꢅ
growth data for the years1975 to 1979
were obtained from the 1996 Regional Eco-
nomic Information System, U.S. Bureau of
Economic Analysis.
generated as Xi1B1 and Xi2B2, respectively.
It follows that a consistent estimate of B3
up to a factor of proportionality may be
obtained by applying probit ML to the struc-
tural equation
Log hourly earning in the local non-
metropolitan area, ln Wiꢀ 21ꢀ N , ipssecified
asa function of individual human capital
attributes(GRADE, AFQT80, EXPER),
gender (MALE), race (BLACK), ethnicity
(HISPANIC), local economic conditions
(AGSHARE, MANUSHAR, COUNEMP,
EMPGR, YPERCAP, POSQML), and
dummy indicatorsof region (NC14, SO14,
WE14). Log hourly earningsupon migra-
tion, ln Wiꢀ 0ꢀ M , are specified as a function
only of individual human capital attributes,
gender, race, and ethnicity, since destination
area characteristics are not observed for
non-migrantsfor the purpoes of generating
predicted initial earningsfrom migration.
Yearsof cshooling, the Armed Forces
Qualifying Test measure of basic skills, and
yearsof experience are all expected to be
positively related to individual stocks of
human capital and, therefore, to hourly earn-
ings. Gender, race, and ethnicity character-
istics are included because females, as well
asracial and ethnic minoritie,shave been
shown to earn less in both metropolitan
ꢃ
ꢄ
∗
ꢀ
ꢀ
(13) Ii = α0 + ln Wiꢀ 0ꢀ M − ln Wiꢀ 0ꢀ N
+ Xi3B3 + εi
with εi denoting the regression error.
The structural equation (13) decomposes
worker i’smigration propenisty
Ii∗ into its
ꢃ
ꢄ
ꢀ
ꢀ
earningscomponent, ln Wiꢀ 0ꢀ M − ln Wiꢀ 0ꢀ N
,
and itscots component,
Xi3B3. Note that
identification of (13) requiresthat Xi1 or Xi2
containsat leats one variable that isnot
included in Xi3.
Data and Variables
The primary data source for the study is
the NLSY, a unique panel of 12,686 indi-
viduals14 to 21 yearsof age in 1979 that
hasbeen reusrveyed annually. Of the full
sample, 1899 individuals resided in a non-
metropolitan county at age 14. Earningsdata
2 Mincer earningsfunctionsoften include a squared experience
term to account for growing depreciation of human capital are
workersage. The qsuared experience term isdropped in this
study because all individuals are close to the date of first employ-
ment. Others(e.g. Lazear) have decomposed returnson experi-
ence into aging and on-the-job training components.
3 Most movesto other non-metropolitan areaswere not to an
adjacent county, as72.4% of the migrantsmoved to a different
multi-county labor market area asdefined by Tolbert and Sizer.