@article{Heckman1990a,
author = {Heckman, J.J. and Walker, J.R.},
title= {The relationship between wages and income and the timing and spacing of births: Evidence from Swedish longitudinal data},
year = {1990a},
doi = {10.2307/2938322},
journal = {Econometrica},
volume = {58},
number = {6},
pages = {1411--1441},
}