First, we know that giving birth for the first time when younger carries a cost in increased infant mortality, but modern medicine and hygiene have strikingly reduced infant mortality, reducing the cost of younger age at first birth. We therefore expect a shift toward earlier reproduction, because the costs previously associated with doing so have disappeared – exactly what we found in Framingham.
首先我们知道妇女生育头一胎的年龄越小，婴儿死亡率的风险也会随之而增加，但现代医学和保健学已经令这个概率大大降低，减少了低龄生育的风险。因此我们都会倾向于更早生育，因为相关风险几乎已经消失了——这也是我们在佛明翰所发现的现象。