Evolution of consanguinity in the Archbishopric of Santiago de Compostela (Spain) during 1900-1979

We present a study of the frequencies of the different types of consanguineous marriages, up to the level of second cousins, using as a source the ecclesiastical dispensations given from 1900 to 1979 in the Archbishopric of Santiago de Compostela (Galicia, Spain). We also report the rate of consanguinity, the average coefficient of inbreeding, and its evolution. From 1900 to 1979, 15,739 consanguineous marriages were registered, corresponding to 25 different categories of relationship. The rate of consanguinity of the total number of consanguineous marriages is 5.13% and the average coefficient of inbreeding is 1.94 x 10(-3), values that are within the wide range of variability found in other Spanish populations. Spain is characterized, with regard to the rest of Western Europe, by a high level of inbreeding with a late and rapid decrease in this factor. The most relevant aspect of the structure of consanguinity lies in the high frequency of marriages between close relatives: 0.16% uncle-niece or aunt-nephew marriages and 1.62% marriages between first cousins, both values with respect to the total number of marriages. This phenomenon appears to be generalized throughout northern Spain. The evolution of the total consanguinity lends itself to a polynomial curve model. The fitted curve of the evolution of the average coefficient of inbreeding has an ascending branch and a descending branch, with the inflection point situated in the year 1918; the regression lines, for both the ascending and the descending branches, have regression coefficients significantly different from 0 (p < 0.001).