There is a widespread misconception that consanguineous marriages, from the Latin consanguineus 'of the same blood' are rare, and essentially confined to remote settlements, or to small religious or migrant communities. In reality, over 1,000 million people world-wide live in regions where 20-50+ per cent of marriages are consanguineous, with first cousin unions especially popular. The common belief that cousin marriage is contracted only in Muslim communities and that it is religiously encouraged also is erroneous. Indeed, consanguineous unions are common in many Buddhist, Christian, Hindu and Jewish communities.
Basic interpretive problems arise in the assessment of consanguinity, mainly because the reasons for contracting a consanguineous union are primarily social and economic, while the outcomes are measured in terms of ill-health. Among recent migrant populations from developing to developed countries a second difficulty is that older migrants tend to evaluate health in the light of their pre-migration experience. But in Pakistan the infant mortality rate is 78/1,000, with undiagnosed infant and childhood deaths experienced by many families. By comparison, in the UK infant deaths are just 4.9/1,000 and subject to diligent medical scrutiny, including autopsy. The point of balance between the social advantages offered by consanguineous marriage as opposed to potential health disadvantages therefore varies significantly in each setting.
Problems also are encountered in assessing the degree to which the adverse health outcomes of consanguinity can be explained in genetic terms. A large majority of the children born to first cousins are healthy. However, a global meta-analysis indicated that early death or major ill-health was on average 4-5 per cent higher in the children of first cousins than equivalent non-consanguineous offspring. Unfortunately, most studies of consanguinity have been conducted in developing countries, often with inadequate planning and design. As an example, factors such as young maternal age at marriage, poor maternal education and short birth intervals are more frequent in cousin marriages. At the same time they are widely recognised to be detrimental to childhood health. Thus widespread failure to control for these variables has resulted in over-estimation of adverse consanguinity-associated health outcomes.
Given the global numbers of consanguineous marriages, comprehensive health-based studies of consanguinity and its short- and longer-term outcomes are over-due, the more so given recent inflammatory comments quoted in the UK media. The prospective 'Born in Bradford' study was launched in 2006 with the aim of following some 10,000 pregnancies and births over 20 years. It is estimated that approximately 50 per cent of the pregnancies will be to couples from the Pakistani community, 50 per cent of whom will be consanguineous. At the international level, the 'Global Burden of Disease' project based on retrospective data from a wide range of countries and continents also has commenced, with the influence of consanguinity as a health risk factor to be assessed across the lifespan.
Both studies are most welcome, and exciting. It is however important to recognize that traditional societies typically consist of a complex series of sub-communities, variously described as clans, tribes, castes or biraderi, within which families have married over many generations. Dependent on their history, through time differences predictably would have arisen in their gene pools by chance, with specific disease mutations present in one or perhaps several sub-communities but absent from the gene pools of others. Stopping cousin marriage will not eliminate genetic disease, since long-term intra-community marriage means that a couple not known to be biological relatives frequently carry the same disease mutation and are at high risk of conceiving affected children.
In the UK Pakistani population, consanguineous marriage and marriage within hereditary social/occupational groups (biraderi) are highly prized as a means of maintaining and strengthening family relationships and honour. However well-intentioned, any call for the abandonment of these well-established marriage customs is simplistic and of limited relevance. A more realistic and workable approach would be to identify the families and biraderi in which specific genetic diseases occur, as an integral component of culture-sensitive genetic counselling and premarital carrier testing programmes.