The Concentration of Reproduction in Cohorts of Women in Europe and the United States

Abstract

Studies in fertility in the industrialized world primarily focus on the decline in average fertility and the determinants of this trend (Bongaarts 2002; UN Population Division 2003; Morgan 2003; Caldwell and Schindlmayr 2003). Variation in fertility within a population is used in an instrumental manner to link birth rates to explanatory variables by means of regressions and de compositions. The recent comprehensive work by Frejka and Sard?n (2004) reviewed changes in completed cohort fertility in industrialized countries and related them to changing age and parity patterns. The present study is interested primarily in the inter-individual diversity in fertility according to the parity distributions of women. This way of think ing about the inter-individual diversity is related to the concentration (or Lorenz) curve, showing what proportion of women have what proportion of children. Such a curve would demonstrate, for example, that 30 percent of American women born in 1931-32 have half of the children born to this cohort, while half of these women have 75 percent of the children. At the same time, 62 percent of children born to this cohort were born to mothers with four or more children, while only 3 percent were born to mothers with one child. These quantities describe something that might be called a division of labor in the matter of fertility (Vaupel and Goodwin 1987). The division of labor concept can be expressed also by an average inter-individual difference in childbearing productivity among women. It can be shown, for example, that the average difference between any pair of women (including women who remained childless) from the 1931-32 American cohort in the number