Darlington became interested in the comparative fertility of cousin marriage after reading Gwen Raverat's Period Piece, a study of the Darwin family. In a letter to the Oxford Magazine, Jan 1958 he appealed to members of the University for family data on the subject; a similar appeal for information in Triangle (Sandoz journal of medical science) was noticed in The Observer and elicited a wide response. The main publication resulting from the data was 'Cousin marriage and the evolution of the breeding system in man', Heredity, 14 (1960), and there were several other shorter articles. Darlington used some of the data and information in The Evolution of Man and Society. Folder comprises letters to the Oxford Magazine, Eugenics Review and Genealogists Magazine.