Commentary: The background and outcomes of the first-cousin marriage controversy in Great Britain.

During the early years of Christianity there were major social and legal differences in attitude towards consanguineous marriage in the Eastern and Western Roman empires, reflecting pre-existing divisions between the Classical Greek and Roman worlds. In Athens and Sparta first-cousin, uncle–niece and half-sib marriages were permissible, with half-sib marriage, and even full-sib marriage continuing within the ruling Ptolemaic dynasty and the settler population of Lower Egypt between the first and the third centuries AD. By comparison, in Rome there was strong disapproval of first-cousin marriage, and the marriage between the Emperor Claudius (41–54 AD) and his niece Agrippina was regarded as especially scandalous. The genetic relationships involved in these consanguineous unions are summarized in Table 1, accompanied by the equivalent coefficients of relationship (r) indicating the proportion of genes shared by each parent, and coefficients of inbreeding (F), a measure of the proportion of loci at which the offspring of a consanguineous union would be expected to inherit identical gene copies from both parents. Not surprisingly, given the period in human history, none of the early judgements on the degrees of permitted and prohibited marriages between biological relatives appears to have had an especially rational scientific basis. However, by the middle of the fifth century the Church had adopted the Roman doctrine on consanguineous marriage, with the initial impact in England recounted by the Venerable Bede writing in the early eighth century. According to Bede, on his installation as the first Archbishop of Canterbury, Primate Augustine had requested advice from Pope Gregory I on Church regulations with respect to first-cousin marriages. The reply from the Pope in 591, citing Leviticus 18:6, was that ‘Sacred law forbad a man to uncover the nakedness of his near kin’. Furthermore, depending on the translation consulted, the Pope advised either that ‘unions between consanguineous spouses do not result in children’ or ‘the offspring of such marriages cannot thrive’. The Papal decision to cite the rather vague but apparently allembracing ban on consanguineous unions in Leviticus 18:6 is noteworthy, since in Leviticus 18:7–18 quite explicit guidelines are provided on the partners a man may or may not take as a wife, with first-cousin unions, and indeed uncle–niece relationships, acceptable. Church permission to marry a biological relative could be sought and granted on payment of a dispensation fee with two different systems used to calculate degrees of consanguinity: the Roman system counted the distance between relatives by summing the number of links from each related individual to a common ancestor, whereas the Germanic system counted the number of links between one partner in the relationship and their common ancestor. Under a canon issued by Pope Alexander II in 1076, the Germanic system was selected as the formal method of consanguinity classification by the Church. This created considerable initial confusion since, for example, a first-cousin relationship (F1⁄4 0.0625) is classified as the fourth degree of consanguinity under the Roman system but the second degree according to the Germanic method. Some semblance of order was restored by Pope Innocent III at the IV Lateran Council in 1215 with the decision that the restrictions on consanguineous marriage applied to third-cousin relationships or closer (F5 0.0039). This level of regulation was confirmed by the post-Reformation Council of Trent (1545–63) and remained in force until 1917 when the requirement for consanguinity dispensation was reduced to couples related as second cousins or closer (F5 0.0156) and in 1983 to first cousins or closer. Somewhat surprisingly, multiple pathways of consanguinity, which often occur in small endogamous communities, were ignored in the latter revision. * Corresponding author. Centre for Comparative Genomics, Murdoch University, South Street, Perth 6150, WA, Australia. E-mail: [email protected]. 1 Centre for Comparative Genomics, Murdoch University, Perth, Australia. 2 Centre for Human Genetics, Edith Cowan University, Perth, Australia. Published by Oxford University Press on behalf of the International Epidemiological Association