In many areas of the world this century, isolates have tended to break up, and particularly since World War II. Sutter and Tabah (1954) were among the first to draw attention to the importance of this process, which they termed ’éclatement’ (bursting) and which they attempted to measure by changes in demographic variables, e.g. endogamy, consanguineous unions. In rural areas of Japan, traditional ways of selecting marriage partners still prevail, but the tendencies to endogamy in various populations have declined year by year.

In 1966 were investigated time trends in isolates based on the frequencies of consanguineous marriages in four different communities in Japan (Yanase, 1966). Because of the complexity of the network of biological relationships in these communities (Table 3.1), analysis was restricted to only those marriages where there was consanguinity corresponding to a coefficient of inbreeding for the offspring of 1/16. It became apparent that the higher the degree of isolation the more rapid was the break-up of the isolate (Figure 3.1).

Such trends in consanguinity may be attributed to demographic variables quite apart from migration, such as a decrease in the mean number of children born, or in the variance (Table 3.2), as well as to a reduced non-random tendency in recent marital practices. In addition, there are significant differences in reproductive performance between native and non-native spouses in these communities.

Human isolates can be divided into four categories:

1. 1. Offshoot populations in areas geographically distant from the main population.

2. […]