晚近，台灣地區總生育率 (total fertility rate, TFR) 的快速下跌引發廣泛討論，總生育率為模擬數值，雖可反映生育水準，卻易受人口年輪 (cohort) 的生育數量 (quantum) 與生育步調 (tempo) 影響造成結果偏誤。於是，不少學者提出調整方法，如Ryder (1959) 透過完成生育率 (complete fertility rate, CFR) 實際值，以年輪平均生育年齡變化，估計較真實的時期別總生育率。爾後，Bongaarts and Feeney (1998) 及Zeng and Land (2002) 使用時期別平均生育年齡差異，直接調整時期別總生育率掌握生育水準的實際變化。本文使用Bongaarts 與Zeng 等人的方法，企圖瞭解台灣地區總生育率下降的原因是生育數量實質減少，抑或只是生育步調延後之故。 研究結果發現，去除生育步調延後 (即平均生育年齡增加) 的影響，1980 年至1997 年間，調整之總生育率均高於觀察值，代表實際生育水準仍高，淨繁殖率 (net reproductive rate, NRR) 亦維持在1 上下。但是，自1998 年起，觀察值與調整值差距縮減，生育步調作用減弱，生育數量實質下滑。此外，台灣地區於2003 年與2004 年的總生育率分別為1230 與1200，兩者皆低於1300，台灣已成為超低生育率 (lowest low) 地區 (Kohler, Billarim and Ortega，2002)。假使總生育率持續下滑，勢必導致人口零成長、負成長的時間提前來到，人口老化速度亦將加快，因此必須及早思考因應之道。

The total fertility rate (TFR) is the most widely used indicator for monitoring the fertility trends. It can be seen as consisting of two distinct parts: (1) a ‘quantum’ component (complete fertility rate, CFR), which equals the TFR that would have been observed in the absence of changes in the timing of births, and (2) a ‘tempo’ component that is attributable to the advancing or delaying of births. The TFR is a hypothetical measure, because no real group of women has experienced or will necessarily experience these particular rates, so many critiques of the TFR involve the changes in the timing of childbearing. Therefore, Ryder (1959) created the concept of CFR and proposed an equation for translating the TFR to CFR by employing a tempo effect, which is composed the mean age of childbearing. Then Bongaarts and Feeney (1998), and Zeng and Land (2002) improved Ryder’s equations to produce more reliable fertility measures. Recently, the TFR in Taiwan has declined more rapidly and pervasively than it was expected. Did a quantum change or the tempo effect cause this? This study applied the Zeng and Land’s tempo free TFR method to the case of Taiwan, for the period from 1980 to 2001. The results show that (1) the adjusted TFRs (CFRs) were higher than the observed TFRs from 1980 to 1997 due to the tempo effect. (2) The net reproduction rates (NRR) were kept around 1 before 1998. (3) The tempo effect has been disappearing since 1998, therefore the adjusted TFRs were close to the observed TFRs and from then on, fertility rates declined in actuality. Since the TFR in 2003 was below 1.3 (1.23), that means we inevitably will face a strictly challenged demographic future.