Under this theory, the opportunity costs-the lost time and income-that parents incur by choosing to have children exert the heaviest drag on the birth rate. The time expended by the mother is the largest single cost.
表一：育齡婦女生育率 (每 1000 人生育之子女數) 年 15 ~ 19 20 ~ 24 25 ~ 29 30 ~ 34 35 ~ 39 40 ~ 44 45 ~ 49 生第一胎平均年齡