Greenwood and Seshadri (2002) show that from 1800 to 1940 there was a demographic shift from a mostly rural US population with high fertility, with an average of seven children born per white woman, to a minority (43%) rural population with low fertility, with an average of two births per white woman.
Greenwood 和 Seshadri (2002年)的研究表明，从1800年到1940年，美国的人口结构发生了变化，从大部分是生育率高的农村人口(平均每个白人妇女生育7个孩子)变成了生育率低的农村人口(43%) ，平均每个白人妇女生育两个孩子。