在“蜥蜴星球”的寓言里，族群固定为200（100雌、100雄），每只寿命100年且每个年龄各只有1雌1雄；每名雌性在24岁生1个雄婴、26岁生1个雌婴、此后不再生育。小行星使24岁与26岁两次出生同时延后10年，人口便以每年净减2只从200降到180，但当生育改在34岁与36岁恢复后，年出生数仍为2只，最终人口又回到200，显示年度指标可骤降而终身生育总量不变。

在人类数据中，常用的总和生育率（TFR）已低于维持人口大致稳定所需的2.1，美国TFR在2007年为2.1、到2022年降至1.6。由于TFR由各年龄组当期出生率加总而得、对“生育时间”（tempo）极敏感并隐含把当下各年龄的生育模式投射到同一女性一生的假设，它可能夸大“生育数量”（quantum）的下滑；相对地，按44岁前累计出生数计算的队列完成生育率（CFR）在过去二十多年并未下降，反而从2000年的1.91小幅升至2024年的1.97。

年龄分布显示更像“推迟而非放弃”：2000年平均26岁已有1个孩子、32岁1.6个、40岁1.9个，而2024年分别为26岁0.6个、32岁1.2个、40岁仍为1.9个，意味着20多岁少生、30多岁补回。再加上当前最常见的育龄为33岁与34岁（源于1990—1991年出生高峰），且34岁女性平均已有1.46个孩子，随着这些大队列逼近40岁，后续出生可能上升；若长期均值接近“略低于2”，人口下降将更缓慢、更可管理，而不如仅看TFR那般急迫。

For 34-year-olds, the average currently sits at 1.46. Large birth cohorts at ages 33-34 (born in 1990-91) could influence these numbers significantly as they approach 40. If the long-term average is near 2 children, this gradual decline in birth rates may reflect changing societal norms rather than an abrupt shift.

In the Planet Lizard fable, the colony is fixed at 200 (100 female, 100 male), each lizard lives 100 years with exactly one female and one male at every age, and each female has exactly two births: a male at 24 and a female at 26. An asteroid delays both births for 10 years, so the population falls by 2 per year from 200 to 180, but when births resume at 34 and 36 the annual total returns to 2 and the population eventually returns to 200, showing a yearly rate can crash while lifetime births do not.

In human data, the total fertility rate (TFR) has fallen below the roughly stabilizing “replacement rate” of 2.1, with America’s TFR dropping from 2.1 in 2007 to 1.6 in 2022. Because TFR is built from current age-group birth rates and is highly sensitive to timing (tempo), effectively projecting today’s cross-age pattern onto one hypothetical lifetime, it can misstate total births (quantum); by contrast the completed fertility rate (CFR), births accumulated by age 44, has not fallen over the past two decades and instead edged up from 1.91 in 2000 to 1.97 in 2024.

Source: Watch who you’re calling childless

Subtitle: Women in America are having as many babies over their lifetimes as they did two decades ago

Dateline: 12月 18, 2025 02:52 下午