In a nationwide survey, the rate of total consanguineous marriages in Japan decreased from 16% to 3% in the period of 25 years from 1947 (Imaizumi et al., 1975). This tendency continued to 1983, as shown by Imaizumi (1986a) in six sample areas representative of all Japan (Figure 4.1). According to Shinozaki (1961), the rate of consanguinity in Japan for the marriage years from 1912 to 1925 was 22.4%, it then slowly decreased to about half that level (12.3%) for the period 1941 to 1945, and after the Second World War a rapid decrease occurred. The inbreeding coefficient (F) in isolated populations in Japan decreased during the period from 1926 to 1963 (Yanase, 1966), though there remained appreciable variation. From the nationwide survey data of Imaizumi et al. (1975) the mean F value in rural areas in Japan (0.0029) was twice that in urban areas (0.0014), and out of eight districts it was highest in Kyushu (0.0034) and lowest in Hokkaido (0.0009). Similar decreases with time have occurred elsewhere, e.g. in Spain the inbreeding coefficient (F) remained constant during the period from 1900 to 1959 and has decreased consistently over the last two decades (Calderon, 1989).
The decline of the consanguinity rate is usually attributed to higher mobility of the population and therefore a wider choice of possible spouse. This paper investigates the effects of marital distance (between birthplaces), religion, socioeconomic factors, marriage form, and opportunity of meeting on the rate of consanguineous marriages in Japan, and also reports the stated reasons for consanguineous marriages: it summarises results reported elsewhere (Imaizumi, 1986a,b,c, 1987, 1988).