The Repeat Frequency of Twins Using Two Sets of Census Data in Japan
Purpose: Couples or families who have two sets of twin pairs need many supports from the society. Then, it is important to know how many such families are living in Japan. It also deals with geographical variations of the repeat frequency of twins.
Methods: Two sets of census data of Japan in 1990 and 1995 were used to estimate the repeat frequency of twin pairs in each family. If there are two persons or more for the same age under 16 years in the same household, these regarded as multiplets. The number was two judged as twins, each of which was in turn treated as a propositus. The next stage was to extract two or more pairs of twins from propositus twin families. The repeat frequency of twinning is the frequency of twinning among families or couples who have already had one set of twins. A denominator was the total number of couples who have 4 or more children and included one set of twins or two sets of twins.
Results: The overall repeat frequency of twins was 0.96% (255/26447) in 1990 and 0.93% (225/24262) in 1995. The repeat frequency of twinning was also computed in each prefecture. The highest rate was obtained in Okinawa Prefecture (1.61%), whereas the lowest was in Oita Prefecture (0.19%).
Conclusions: The overall repeat frequency was 0.96% in 1990 and 0.93% in 1995. Reasons of a lower value of repeat frequency of twinning in Japan were related to small numbers of sibship size in recent years and lower twinning rates in Japanese compared with European population. Another reason is also related to limitation of census data.
Acknowledgements: I am grateful to the staff of Statistics Bureau and Statistics Center in Japan.