Families come to us with an average income of about 12,000 dollars a year and an average family size of four.

来找我们的家庭年平均收入大约为1.2万美元，平均家庭人数为四人。

But as child mortality rates declined, and the US industrialized, the average family size declined from 7 children in 1800 to 3.5 children by 1900.

但随着儿童死亡率下降和美国工业化，平均家庭规模从 1800 年的 7 个孩子下降到 1900 年的 3.5 个孩子。