Evolution of consanguinity in the Bishopric of Lugo (Spain) from 1900 to 1979

The levels of consanguinity in human populations are known to be influenced by demographic and geographical factors, and by socio-cultural customs that govern spouse choice. Within the general model applicable to the populations of Western Europe, Galicia (north-west Spain) is a region that is especially interesting, due to its geographical and historical peculiarities that could have conditioned its behaviour pattern with respect to consanguinity.

The object of this paper is to analyse the evolution of the consanguinity structure levels in the rural population of the Bishopric of Lugo (Galicia) during the period 1900-1979.

The study covers a total of 752 parishes included in 52 municipalities of the rural area of the Bishopric of Lugo. Of a total of 117,583 marriages in the period studied, 6701 were between biologically related individuals up to the level of second cousins. The percentage of the different types of consanguineous marriages, the consanguinity rates, and the average coefficient of inbreeding of the population was calculated. Also, for the analysis of the temporal variation of consanguinity, version 3.01 of Jandel Scientific's Table Curve program was used.

The consanguinity rate for the whole of the period is 5.70% and the average coefficient of inbreeding is 2.2477 x 10(-3), values which fall within the observed range for Spanish populations. As for the consanguinity structure, the most frequent types of consanguineous marriages are between second cousins (2.65% of all marriages), but the high proportion of marriages between uncles/nieces or aunts/nephews (0.16%) and first cousins (2.12%) with regard to the total number of marriages should be emphasized. The average value of the M22/M33 ratio is 0.80, and throughout the study period it was above the 0.25 expected in conditions of panmixia. Regarding the evolution of the average coefficient of inbreeding, its annual values fit a polynomial regression curve with one ascending and one descending branch and an inflection point in the year 1919.

As in other rural areas of Galicia, the structure of consanguinity in the Bishopric of Lugo is characterized by the high proportion of marriages between uncles/nieces or aunts/nephews and first cousins, a phenomenon which appears to be generalized in the rural populations of northern Spain. It is suggested that the high proportion of uncle/niece marriages that have been found in the population studied are very probably due to the important migratory phenomenon which took place in this region at the end of the 19th century and in the first third of the 20th century.