Human inbreeding: a familiar story full of surprises
Introduction In most Western countries, especially those located in N orthern Europe, North A m erica and A ustralasia , m arriages betw een close biological relatives generally are regarded as both highly undesirable from a social perspective and dangerous in term s of the physical and m ental well-being of their progeny. By comparison, in North and sub-Saharan Africa, and West, Central and South Asia consanguineous unions are widely contracted, and in some communities they account for a majority of m arital unions in the current generation (Bittles 1994).There appears to be no particular rationale for this sub-division of hum an populations into opposing forms of m arriage preference and, as illustrated in Table 6.1, even w ith in the m ajor religions th e re are qu ite m arked differences in a ttitu d e to close-kin m arriage. Thus, in C hristianity, the O rthodox churches prohibit consanguineous m arriage, and the Rom an Catholic church requires Diocesan permission for m arriages between first cousins, and until early in the present century this ruling included second-and third-cousin unions. According to the Venerable Bede (731), the basis for the Latin church’s guidance on acceptable levels of consanguinity within m arriage was originally stated in a communication by Pope Gregory I to St Augustine in 597, ‘experience shows that such unions do not result in children, and “sacred law forbids a man to uncover the nakedness of his kindred” ’, the la tte r com m ent being derived from the book of Leviticus 18:6. However, following the Reformation, the Protestant denom inations opted to use as th e ir guidelines a restric ted version of the deta iled Judaic regulations established in Leviticus 18: 7-18 and to sanction marriages up to and including unions between first cousins, equivalent to a coefficient of inbreeding in the progeny (F) of 0.0625.A sim ilar degree of non-uniformity exists in Hinduism. The Aryan Hindus of north India impose a prohibition on m arriage with biological kin that extends back approximately seven generations on the male side and five generations on the female side (Kapadia 1958). By com parison, for the Dravidian Hindus of south India m arriage between first cousins of the type
m other’s b ro ther’s daughter (MBD) is strongly favoured and, particularly in the states of A ndhra Pradesh, K arnataka and Tamil Nadu, uncle-niece m arriages (F = 0.125) also are widely contracted.In general, Muslim regulations on m arriage parallel the Judaic pa tte rn prescribed in Leviticus, but, although uncle-niece unions are perm itted in Judaism , a form of m arriage that was not adopted by the Reformed Protestant churches, they are forbidden by the Koran. Yet double first-cousin m arriages (F = 0.125) are recognised within Islam. In South Asia, Buddhism sanctions m arriag e betw een firs t cousins w hereas the Sikh relig ion, which also originated in north India, follows the Aryan Hindu m arital tradition and forbids consanguineous m arriage, although in some m inority Sikh groups there may be a relaxation of this proscription.There is a similar lack of coherence in the legislation that has been enacted in different countries to govern perm itted types of m arital relationships. Thus while first-cousin m arriages are legal in the U nited Kingdom and Australia, they are crim inal offences in eight of the states of the U nited States and illegal in a further twenty-two states (O ttenheim er 1990). Exceptions can be incorporated into state laws, for example, to perm it uncle-niece m arriage within the Jew ish com m unity of Rhode Island (B ratt 1984). Legislation approved and adopted at the national level may also prove to be inoperable in practice, as exemplified by the Hindu M arriage Act of 1955, which includes a ban on uncle-niece m arriage (Kapadia 1958). Yet in a study conducted between 1980 and 1989 in Bangalore and Mysore, the two m ajor cities of the state of K arnataka in south India, 21.3 per cent of Hindu m arriages were uncle-niece unions (Bittles et al. 1992).