The inbreeding coefficients (F) were estimated according to marital distance between birthplaces and the opportunity of encounter. As for marital distance, the F-value was the highest for the shortest distance (0.00252), whereas the F-values were similar in the other distance classes (0.00072–0.00097). As for opportunities of encounter, the F-value was 30–37 times higher in the group for friendship from the time of childhood (0.00986) than in those for school, workplace and social gatherings (0.00033) and for chance meetings (0.000