Commentary: Of the same blood.

These selected respondents reported a very high incidence of first-cousin marriages in their families. A smaller proportion of marriages were with more distant cousins, but Pearson remarked that second and third cousins in these families were also often related in more than one line. He lumped them all together and concluded that 'consanguineous marriages in the professional classes probably occur in less than 8% and more than 5% of cases'. Yet, only 1.3% of patients in the Great Ormond Street Hospital for Children were the children of cousins. Pearson concluded that 'the diseases of children are not largely due to any consan-guinity between their parents'. 21 Endorsed by the Darwinian establishment, George Darwin's conclusions reassured many people whose family trees featured marriages between cousins. Englishmen could also rest more easy when they considered that Queen Victoria was married to a first cousin, and that several of her descendants had also married cousins. And Darwin's conclusions seemed only common sense to landowners in the House of Lords, who knew that the inbreeding of good stock was sound policy. All human beings are related, and some are more closely related than others. In medical and demographic literature, consanguineous marriage is usually defined as marriage between a man and a woman who are related as second cousins or closer. 1 First-cousin marriage is supposed to be the most prevalent form globally. Effects on disease and death have been demonstrated primarily among children of