但在过去30年终，美国白人和黑人生育率都远低于替代率，三十年来美国生育率的起落主要原因是西班牙裔生育率较高，这反映了一个常有的现象：初代移民生育率往往与较贫穷的来源国相仿。
But throughout the last 30 years, fertility rates for white and black Americans have remained significantly below replacement level, and the three-decade rise and fall of US birth rates is explained primarily by higher Hispanic fertility, reflecting the common phenomenon that first-generation immigrants’ fertility rates are typically similar to those in their poorer countries of origin.