Among 139 marriages from which resulted 507 children, 187 of the children were born deaf ("rubella children," etc., excluded). These deaf children were all pupils of the Royal Institute for Deaf Mutes. In 17 of the marriages the parents were related to each other. They had 85 children, of whom 28 were deaf. In 11 cases the parents were full cousins (Families 1, 2, 3, 4, and 5), in 1 marriage twice full cousins (Family 6), in 1 full cousins and also second cousins, and in 4 second cousins (Family 7). These 17 consanguineous marriages represented 12.2% of the total 139 marriages; if we take into consideration only the 13 first-cousin marriages, the proportion is 9.4%, with a standard deviation of ±2.5%. In the entire population of Holland, the proportion of consanguineous marriages is about 0.16% (Polman1).
Earlier investigators found proportions of 6.47% (De Wilde2) of consanguineous marriages