The central concept of microeconomic theories of fertility is opportunity cost—the product of wife’s employment lost due to childbearing and the value of her employment. This paper presents a model for analyzing opportunity cost using panel data. The average loss of employment attributable to a second- or higher-order birth, calculated at around age 2, is over 400 hours per year. This time cost represents an income loss of about $1050 in 1969 dollars. Time cost is independent of such demographic factors as birth order and age of oldest sibling. Neither does time cost depend on husband’s wage rate or wife’s education or potential wage rate. This indicates that many microeconomic models of fertility have been seriously misspecified. The paper also compares results from static and dynamic models, explores possible problems due to simultaneity bias, investigates the relationship between changes in employment (including time cost) and initial employment level, and identifies the difficulties of theorizing about opportunity cost.

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