There are fewer than 30 confirmed patients known to suffer from this disease; 22 of them are from the Netherlands, and of these, 14 are from Urk specifically. The area of Urk was for centuries an island in the
Zuiderzee until in 1942 the surrounding seafloor was
reclaimed. Despite the resulting facilitation of residents' access to other areas, cultural factors originating in the area's former isolation have led residents to maintain a mostly closed community through practices including nearly uniform
endogamy. These practices have both short-run and long-run effects significantly raising the average degree of
consanguinity between pairs of potential parents: At any given time, the exclusion of less closely related persons from the set of potential alternative partners increases the average degree of consanguinity across those matches that are formed. In the long run, it not only continues that same effect across generations but effectively halves the community population's growth rate relative to that of communities that welcome outsiders as childbearing partners and accept into the community even children having only one native community member as a parent. This combination of effects has increased the intra-community incidence of certain
hereditary disorders, including Van Buchem's; of carrier status for the
autosomal recessive conditions among those disorders; and of some physical traits not clinically regarded as anomalous. == See also ==