Secondly, the family as a small group serves as buffer to its members to lessen the social and economic impact of population ageing superimposed upon it as if from outside before reaching individual members. The present chapter deals mainly with the former aspect, but consideration is also given to the latter.
Population ageing has very important bearings on the changes in the number and structure of the family and household. Let us first discuss how much population ageing is attributed to determine the size of the family and household. Then, an attempt is made to look into the course of transformation in which dynamics in population ageing affect the structure of the family and household. Here the term "structure" mainly means the composition of the household by family type and patterns of co-living of the elderly with their offsprings.
B. Change in the Size of Household
The recent demographic changes in Japan have been rapid and have made quite considerable effects upon the family size and structure. Obvious changes occur in the average size of household and family in Japan.Table 9 represents trends in average household size for Japan.
It is apparent that the household size has shrunk down considerably in the long run during 65 years' time span, but some may wonder why the process of shrinkage in the household size has been so slow in comparison with the fertility decline occurred roughly at the same time. For example,total fertility rate had substantially been reduced from 3.65 in 1950 to 2.00 in 1960. On the other hand, however, the average size of household diminished from 5.02 to 4.52 only by 0.50 in term of persons per household between the same years. By the same token, for the Republic of Korea,while total fertility rate was reduced from 4.47 to 3.33 between 1970 and 1975, the averge household size shrinked from 5.2 to 5.1 only by 0.1 person per household. It seems that there always exist some kind of demographic lags, so that the process of reduction in household size does not necessarily follow the trend in decline in fertility in an immediate sequence.Part of explanation may be sought in the mortality reduction taking place at the same time, the mortality reduction which facilitates enlarging the household size in a completely opposite direction to the effect of fertility decline under the circumstances where fertility does not drop. The substantial reduction in child mortality occurring in the early cycle of demogaphic transition precisely means an increase in fertility in Japan. Between 1920 and
