[In this paper, the authors develop a simultaneous equation model of birth rates composed of four estimated equations. This work differs from past research in that it considers the simultaneous relationship between birth rates and income and includes the cost of fertility as an explanatory factor. This cost is measured by the female labor participation rate under the assumption that income foregone due to fertility is a significant opportunity cost.]

Paul R. Gregory, John M. Campbell, Benjamin S. Cheng, A Simultaneous Equation Model of Birth Rates in the United States, The Review of Economics and Statistics, Vol. 54, No. 4 (Nov., 1972), pp. 374-380