Parents, some say, are people who use the rhythm method of family planning. One might better say that parents are optimists, people who think that the present is good and the future probably better. People who look forward with confidence often have an extra child; those who think that their situation may worsen are cautious about increasing family size.
In fact, most people want children, but not more than can be raised well given family standards. When people believe that economic prospects are brightening, they marry early and plan to have children quickly, a phenomenon that I call the fertility opportunity effect. The perception of expanding opportunity explains the various episodes of high fertility in America.
In the colonial and frontier periods, for example, the natural wealth of the American continent invited population increase, and, indeed, very large families were common. Settlers in the New World averaged much higher fertility rates—that is, more children per woman — than were usual in the societies from which they came. Numerous authors, beginning with Adam Smith, attributed the very large family size typical in the colonies to the seemingly boundless natural resources that required human labor for transformation into wealth.
The transition from high to low fertility followed the frontier as it swept westward. Free land vanished, and good land became expensive as isolated homesteads...