The reason that trend toward smaller families spread was that this whole way was driven by an idea -- the idea that couples can exercise conscious control over how many children they have.
倾向小家庭的趋势之所以会蔓延， 是因为这个方式是 由一个想法引起的—— 那个想法就是夫妻们有权力 刻意地掌控家庭人口数量。