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Consanguineous marriage is a union between couples who are related as second cousins or closer.
In Pakistan, consanguineous marriage is reported to be higher than 60% of the population in 2014. Also, BBC reports that an estimate of at least 55% of British Pakistanis are married to first cousins, and that British Pakistanis are 13 times more likely to have children with genetic disorders than the general population. They account for just over 3% of all births but have just under a third of all British children with such illnesses.
While Italy as a whole has low rates of consanguine marriages, the late geneticist Luigi Luca Cavalli-Sforza published detailed work, based on both blood typing and parish records, showing relatively high rates of consanguinity on Sardinia and Sicily, and locally in remote pockets of mainland Italy.
A 2013 study cited high rates of single-gene diseases in the Eastern Mediterranean due to cultural practices.
Consanguine marriages are often the consequence of arranged marriages. For example, in Pakistan and Bangladesh, marriages are typically arranged by relatives and there is a strong preference for consanguineous marriage.
It has been suggested that consanguinity may be a relatively cheaper way for families to deal with the problem of dowry costs in rural marriage markets, that is, more stringent credit constraints and lower wealth levels will lead to lower dowry payments and a higher prevalence of consanguineous marriages.
Female education may affect consanguineous marriage in at least two ways. Better-educated women demand more freedom in choosing a marriage partner (female empowerment hypothesis), and going to school prevents women from fulfilling spousal obligations, especially since consanguineous marriages tend to be contracted at a relatively early age (role incompatibility hypothesis). According to a 2017 study on Muslims in Israel, the rise in the age of leaving school explains about a third of 60%-decline of consanguineous marriage between 1975-1979 and 2005-2010. Similarly, a 2013 study on consanguineous marriage in India concluded that a larger proportion of less educated women (35%) are married to their blood relations compared to women with higher education (11%).
Education may also reduce consanguine marriages by raising awareness about the health risks of consanguinity. A 2015 study on Saudi Arabians found that the odds of a positive attitude towards consanguinity were 50% less in respondents who received health information on consanguinity compared to those who had not received health information about consanguinity.
In the Bible there are many examples of consanguine marriages. After his father Isaac recommended him to do so (Genesis 28:2), Jacob married his cousin Rachel (Genesis 29:1). Later, Jacob also married his other cousin Leah (Genesis 29:1).
In Catholicism, marriage is invalid between persons related by consanguinity in all degrees of the direct line and that in the collateral line, marriage is invalid up to and including the fourth degree. In particular, first cousins cannot marry in the Catholic Church. However, this last rule is seen more as an "ecclesiastical law" instead of a "divine law", and a special dispensation permits two first cousins to marry validly in the Catholic Church.
Consanguineous marriage is widely favoured in a large majority of the world's Islamic populations. In particular, many Arab countries display some of the highest rates of consanguineous marriages in the world ranging around 20-50% of all marriages, and specifically favoring first cousin marriages with average rates of about 20-30%. In Islam marriages between first cousins are not forbidden. Two additional factors specifically apply in Islamic society: the belief that in contracting a first cousin marriage of the type father's brother's daughter, the marriage partners benefit from the comparability in status of their respective fathers, and the fact that two of the wives chosen by Muhammad were biological relatives, and he also married his daughter Fatima to Ali, who was his ward, with the son of his paternal uncle.
Consanguineous marriage increases a couple's chance of both carrying the same recessive disease variant and affecting their children. The effect is particularly marked for rare disorders, because a carrier is unlikely to find a partner who carries the same disorder unless they are related. Consequently, in communities in which consanguineous marriage is common, there is an increased birth prevalence of many rare recessive conditions, and a significant increase in the total birth prevalence of congenital and genetic disorders.
In populations with a high rate of consanguinity, there is a significant increase in the prevalence of common adult diseases like cancer, mental disorders, heart diseases, gastro-intestinal disorders, hypertension and hearing deficit.
A 2018 study on 363.960 individuals born in Northern Ireland shows that being a child of consanguineous parents is associated with having an increased likelihood of psychotropic medication use in adulthood. Precisely, children of first-cousin consanguineous parents are more than 3 times as likely to receive medications for common mood disorders and more than twice as likely to receive medications for psychoses compared with children of unrelated parents. Also, a positive correlation between consanguinity and schizophrenia has been observed.
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- Islam Online; Muzammil H. Siddiqi, Islamic View on Marrying Cousins. (archive snapshot)
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