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I am delighted to have this opportunity to discuss the ideas in my book. I would like to thank Amy M. Froide, Claudia Goldin, Jane Humphries, and Pamela Sharpe for thoughtful comments that highlight the main ideas of the book. I agree with many of the points they raise but, naturally, not with all of them. I hope that readers will find these discussions interesting enough to find out more about my position by reading the book.

While most of my responses are addressed to individual commenters, I would like to start by discussing an issue that appears in two of the comments. Both Froide and Goldin ask why the gender wage gap persisted into the twentieth century. I have two answers to this question. First, while the importance of strength has faded, gender differences in child rearing remain important. In spite of the lower fertility of the twentieth century, women still experience greater penalties for childbearing than do men. Jane Waldfogel (1998) notes that the "family gap," the wage gap between women with children and women without children, grew during the 1980s. Unfortunately, I do not believe this portion of the wage gap will disappear. Fertility has fallen, but there is a limit to how much more it can fall without leading to the end of the human race. I feel confident in predicting that pregnancy and breastfeeding will continue to be tasks of the mother. There is room for improving gender equity in child care, but I do not think we will ever reach complete equality.

Second, and perhaps more importantly, wage discrimination appeared about the time that strength became less critical. The beginning of the twentieth century saw the rise of internal labor markets, which were structured to favor men. The rise of firm-specific capital increased the cost of turnover, and as firms developed incentive systems to reduce turnover, they found that men responded better to wage structures that rewarded higher tenure, and they provided men with opportunities for increasing wage profiles without providing women with the same opportunities. This trend, which Goldin (2006) discusses, prevented women from benefiting from the declining importance of strength. In contrast to the family gap, this portion of the wage gap might disappear, and I think it has been disappearing. Thus wage discrimination has prevented the gender wage gap from falling as much as might be expected if strength had been an important cause of the wage gap in the nineteenth century. However, since wage discrimination is itself declining in importance, I expect to see the gender wage gap become smaller, if not zero, in the future.

Froide

Froide and I disagree on some fundamental assumptions about markets, but in some cases our apparent differences are simply the result of terminology. For example, Froide and I seem to use different definitions of the word productivity. Froide notes that "if we chose to measure women's productivity by including their household work and child care alongside their market labor, I think we would find that women were as productive as, or more productive than, men in the past." This implies that productivity should include all the benefits that individuals provide for society. My definition of productivity is more narrow. By productivity I mean only the value of the marginal product of labor, or the increase in firm revenues that results from hiring one more worker. Productivity, by this definition, does not include any benefits to anyone outside the firm and certainly is not a measure of a person's value to society. I agree that housework and child care are valuable, but I do not count these as part of the marginal product of the worker relevant to the employer. When economists say that quarterbacks earn more money than nurses because they have higher productivity, we do not mean that quarterbacks are more valuable to society. We simply mean that they earn more profits for their employers. If we are studying wages, though, I believe that the value of the marginal product is the definition we should use. Economists believe that wages are determined by the value of the marginal product, not...