「世代」是一個是父死子繼的過程，亦是一個家庭生命歷程的交換、更新與取代，要產生人口「替代」（replace），就必須有人口「再生產」或「繁殖」（reproduction），才會有所謂的替代過程發生。本文以繁殖率（reproduction rate）為基礎，計算世代長度（mean length of generation）。並以臺灣總生育率的變化作為切入點，從其半世紀以來的整體趨勢，去看生育率變化與世代長度變遷之關連性；而代間間隔時間之變遷必然影響臺灣家庭世代的共存結構，使得家庭結構產生改變。本文從女性觀點著手，納入初婚年齡中位數與平均餘命的概念，討論半世紀以來臺灣家庭世代共存結構變遷。世代變遷之所以重要在於其不只是對家庭產生意義，也同時影響個人生命及其生活之樣貌。 在1979年以前，臺灣世代長度變化與總生育率變化的趨勢大抵一致，在1979年以前，世代長度隨著總生育率下滑，到了1979年，總生育率還是不斷減少，但世代長度卻已停止下降，向上攀升。 從家庭可能產生之人口數量說明臺灣五十餘年之家庭世代共存結構變化： 1) 50年代--三代共存：此時期的家庭人口相當多，可能同時包括雙親、七個子女、七個媳婦，八至十個孫子三代，家庭裡共存人數可能多到40人以上，此時期家庭最大的特色在於叔姪同齡、「長兄如父、長嫂如母」的特殊現象。 2) 60年代--四代共存：從人口層面推論，四代同堂最有可能發生在70年代，主要在於此時期的平均餘命延長，讓親代有足夠的生命等待曾孫子女的來臨，同時也有足夠的生命看著所有的子女長大結婚生子。 3) 70年代--四代共存：此時期的家中共存人數較前期減少許多，約莫在30人左右，主要原因是此時期的總生育率已下降至1.9人。 4) 80年代--四代共存：和70年代相同的是，此時期也可能是個四代同堂的家庭型態，不同的是，由於總生育率下降，家庭共存人口比起70年代數量驟減，約莫10-15人。 5) 90年代--三代共存：此時期的家庭結構將再度回到三代共存的情形，與50年代三代共存不同的是，此時期由四代共存回到三代共存原因在於遲育現象，而家庭人口組成也愈趨簡單。 6) 21世紀--兩代共存：此時期的家庭結構將產生很大的變化，結婚年齡延後加上所生育的子女數銳減，使得家庭人口數將更少，可能出現僅有兩代共存的情形，勢必造成親代與子代的鍊結更深，意味著所有雙親照顧的責任可能全落在一個子女身上。 世代變遷影響下的家庭世代共存結構改變，改變了家庭人口的規模與組成，進而影響了家庭成員的生活模式及型態；現今家庭所生育子女數僅1人餘，因此獨生子女現象造成一人需負擔兩人的照顧問題，無論是在經濟或心理上都將是一種沈重的負擔。 關鍵字：世代、代間間隔時間、女性生命週期、家庭世代共存結構 / The languid flow of one generation to the next symbolizes the constant reweaving of our social fabric: As daughters assume the roles their mothers left in death, the life of the family is renewed and perpetuated, but also is steered onto a unique path. The motivating force behind this familiar familial story is the reproduction of human life, without which the replacement of human populations, of mothers with daughters and fathers with sons, cannot occur. Naturally, reproduction rates form the crux of my research, as I use it to calculate mean generation lengths over the last fifty years. The trends and trajectories of the past half-century are integral to examining the interconnectivity of changes in the total fertility rate and changes in mean generation lengths; moreover, changes in mean generation lengths impact significantly the structure of intergenerational coexistence in particular and the entire family structure in general. My research also approaches the topic through a yoni-centric perspective: I employ statistics concerning the median age of first marriage and average life expectancy of women to discuss the evolution of the structure of intergenerational coexistence in Taiwan over the past half-century, as women, stereotypically speaking, exhibit more predictable and stable life patterns than men. Ultimately, generation replacement is important not only because it fosters meaning within the family, but also because it weighs heavily on the very content that forms the lives that individual family members lead. Prior to 1979, the mean generation length and total fertility rate in Taiwan exhibited similar fluctuation patterns: Mean generation lengths shortened in accordance with total fertility rate’s steady decline. However, by 1979, the total fertility rate continued to fall, while mean generation lengths leveled off and even began to rise. Through my research, I discovered that potential family size serves as an effective analytical window to study the evolution of the structure of intergenerational coexistence in Taiwan over the last fifty odd years: 1) 1950s (three generations coexisting): The family size during this decade was very large, and can include parents, seven children, seven daughter-in-laws, and eight to ten grandchildren all living under the same roof. Total family size sometimes exceeds forty persons. There were two unique characteristics of families during this decade. First, family members of two generations may be of the same age (i.e. an uncle is the same age as his nephew). Second, traditional practice dictated that upon the death of the parents, the eldest son assumes the role of the father (head of the house) and his wife assumes the role of the mother. 2) 1960s (four generations coexisting): According to population studies, the phenomenon of four generations living under the same roof was mostly likely to occur during the 1970s, since average life expectancies increased significantly during this decade, allowing parents to witness grandchildren marry and sire great-grandchildren. 3) 1970s (four generations coexisting): Due to total fertility rates declining to 1.9, family size during this decade decreased significantly, consisting of at most about thirty persons. 4) 1980s (four generations coexisting): Like the 1970s, it was also possible for four generations to live under one roof during this decade. Unlike the 1970s, family size shrunk to about ten to fifteen persons in accordance with steady declines in the total fertility rate. 5) 1990s (three generations coexisting): Family structures returned to three generations living under the same roof during this decade. Unlike the 1950s, however, the cause of this decline was the fact that women began bearing their first child at an older age, which resulted in simpler organization of family members. 6) Present (two generations coexisting): Family structures are undergoing dramatic changes in the 21st century. People are marrying later in life and having fewer children, which leads to considerable decline in family size and only two generations living under the same roof. This, however, has also precipitated closer ties between parents and their children, and the responsibility of caring for both parents in their old age is likely to fall on a single son or daughter. The structure of intergenerational coexistence has evolved over the past half-century under the influence of changes in the process of generation replacement. As a result, the roles of individual family members and the particular burdens they bear have also undergone considerable change. Today, families often have only one child, which results in the problem of a single child having to care for two aging parents. This is a mighty burden in terms of both economic and emotional sacrifice. Keyword: Generation、Mean Length of Generation、Life Course of Female 、Structure of Intergenerational Coexistence