From 626 ascendant genealogies, known as ‘birth briefs’, deposited by members of the Society of Genealogists in their London library, rates of consanguineous marriage and coefficients of mean inbreeding (α) of offspring were estimated for cohorts of marriages contracted in the late nineteenth and early twentieth centuries. The rate of first cousin marriage in the generation estimated to have married during the 1920s was 0·32%, with no marriages between second cousins. The mean inbreeding coefficient for the offspring of these marriages was estimated as 0·0002. In the previous generation 1·12% of the marriages were between first cousins, and the estimate of mean inbreeding was 0·0007. Comparison with data taken from the published literature suggests that the levels of cousin marriage observed are consistent with a secular decline during the late nineteenth and twentieth centuries.