Tim Dieppe outlines the case against cousin marriage
Introduction
Cousin marriage was banned in Britain for a thousand years until the time of King Henry VIII. Then King Henry VIII famously broke with Rome in order to marry Anne Boleyn. Later, in 1540, Henry married Catherine Howard, his fifth wife, who was a first cousin of his second wife Anne Boleyn. To do so, he had to change the law, since marrying such a relative was prohibited and considered incest – even though Catherine was not a blood relative of Henry’s.
The law allowing cousin marriage has remained in place in the UK ever since. The Church of England has a table of Kindred and Affinity which shows all the relations which are prohibited for marriage. Cousins are not listed, meaning that cousin marriage is allowed.
Last year Richard Holden MP proposed a bill to prohibit marriage of first cousins. The bill was presented in parliament in December 2024 when he gave a speech on the issue. Iqbal Mohamed MP opposed the bill, noting the popularity of cousin marriage in some cultures and suggesting genetic testing of prospective couples instead. The issue of cousin marriage was also debated in Parliament in June 2025 in a Westminster Hall debate also proposed by Richard Holden.
The Church banned cousin marriage before
We are so influenced by what the law is where we live and grow up that we often don’t appreciate that it hasn’t always been this way. Joseph Henrich, in his influential book: The Weirdest People in the World (Penguin, 2020), argues that the Church’s insistence on monogamous marriage and the banning of cousin marriages was absolutely crucial to the success of the West. The acronym WEIRD stands for Western, Educated, Industrialised, Rich and Democratic. Without such laws on marriage, the West would not have built such successful societies. His argument is compelling and has been well received.
An appendix to the book lists dozens of rulings against cousin marriage by the Church. For example, the Synod of Trullo in 692 AD formally forbade marriage to first cousins and corresponding affinal kin. In 741 AD, Emperor Leo III made it illegal to marry second cousins, and later second cousins once removed. The Synod of London in 1075 banned marriages up to sixth cousins, including affinal kin. Several other councils reinforced this ruling. This forced people to look outside their families or clans to find partners, with huge benefits for society.
Benefits of banning polygamy and cousin marriage
The benefits of monogamous marriage are well known. It benefits both the spouses and any children. It is foundational for a well-functioning society. Where monogamous marriage is the rule, in theory every male can have a partner. Where polygamy occurs, however, some men get several wives leaving other men with no partners. Even a mild amount of polygamy means a large proportion of males become forced bachelors. These men are thus disenfranchised and disgruntled. They are very likely to become violent and disruptive to society as a result, hindering the progress of society.
The Church’s banning of cousin marriages in the Middle Ages had its own effect: clan systems were completely destroyed. Prior to this, most people married cousins in order to preserve the clan and incentivise loyalty to the clan. Clan loyalty trumped all other demands for loyalty. Marriages to cousins were the norm and were generally arranged by clan authorities. However, once marriage up to sixth cousins was banned, then you were forced to find a partner outside your clan. What Henrich calls the “accidental genius of Western Christianity” (p161) therefore undermined clan authority. It meant that couples set up their own independent homes, and became more loyal to the Church and later to voluntary organisations and to the nation as a whole, rather than to the clan. This came with huge benefits to society in general. Today clannism is virtually non-existent in Western societies, except where it has been imported from other cultures.
Does the Bible allow cousin marriage?
The Church in the Middle Ages became somewhat obsessed with concerns about incest. This concern was defended by reference to Leviticus 18:6 which forbids sexual relations with close relatives. In context, however, while marriage with close relatives such as brothers and sisters was clearly forbidden (Lev 18:7-18), relationships with cousins are not mentioned, leading commentators to conclude that cousin marriage was not forbidden in Mosaic law. In Genesis, Isaac married his cousin Rebekah and Jacob married his cousins Leah and Rachel. However, these examples may not be seen normatively, just as polygamous marriages in the Old Testament are not regarded as normative by either Jews or Christians.
In fact, Abraham married his half-sister Sarah (Gen 20:12). Abraham’s brother Haran appears to have married his niece Milcah (Gen 11:26, 29). Adam, of course, married Eve who was effectively his twin sister. His children would have married their siblings. No one today views these relationships as normative since they are expressly ruled out by Mosaic law (Lev 18:6-18). One can argue that genetic deterioration had not really progressed enough to make it a problem to marry your sister or other close relative in Abraham’s time, whereas by the time of the Exodus the progress of mutations over several generations meant that this was more likely to cause genetic problems.
The law on which relatives you could marry in the Bible therefore changed over time from allowing sibling marriage to banning sibling marriage and marriage to some other close relatives. This means that there is a case for arguing that these laws are more flexible and adjustable than other laws such as the ten commandments. The stricter prohibitions on marrying close relatives in Leviticus arguably reflected the genetic deterioration over time, making sibling marriage a health risk by the time of the Exodus. There have been many more generations of mutations since then meaning that cousin marriage is now a significant health risk as will be discussed below. It would seem an absurdly legalistic or wooden application of the text to argue that cousin marriage should still be allowed because strict interpretation of Leviticus allows it, even though these laws changed over time.
Prevalence of cousin marriage
The term ‘consanguineous marriage’ is used by clinicians to describe marriage between relatives who are related as second cousins or closer. Assuming there has not been multi-generational consanguineous marriage, double-first cousins share 25% DNA, first cousins 12.5% DNA, and second cousins 3.125% DNA. These percentages will be significantly higher where there have been generations of consanguineous marriages.
Globally, around 80% of all marriages in history are thought to have been consanguineous.[1] Currently, around 8.5% of births are to consanguineous parents. 10% of the global population practises consanguine marriage, and 20% live in communities where it is practised. It is most prevalent in the Islamic world where clannism is still strong. Countries with the highest prevalence of consanguineous marriage include: Kuwait (52%), Pakistan (51%), Nigeria (51%), Sudan (50%), Bahrain (44%), and Qatar (44%). Prevalence in the West is much lower at less than 1%.
Where clannism has been imported through immigration, rates of consanguineous marriage can be a lot higher. Amongst British Pakistanis it is reckoned to be between 40 and 60%, but over 90% amongst some of the clans or biradaris in Bradford. Consanguinity is thought to be an underreported phenomenon so available statistics may understate its prevalence. As a generalisation, it is true to say that most Muslim populations are highly consanguineous and most non-Muslim populations are not.
Medical problems with cousin marriage
The medical problems with cousin marriage today are well known. The risk of inheriting a serious genetic disorder for children of unrelated parents is 2-3%. For first cousins the risk doubles to 4-6%. This is roughly equivalent to the risk faced by children of mothers over 34 years of age. However, this percentage risk is much higher for children of cousin parents where the community has been engaging in endogamous marriage for many generations. In this case the risk of inheriting a serious genetic disease increases to over 10% – three or more times the standard risk. There are also other health implications such as increased susceptibility to various cancers and infections, increased risk of psychoses such as schizophrenia and Alzheimer’s, higher infant mortality rates, and depressed IQ scores. A recent study showed that there are more infant deaths in the UK linked to cousin marriage than there are to substance abuse. In short, there are serious medical risks from cousin marriage because of the levels of shared DNA meaning that shared harmful mutations are much more likely.
Harms of clannism
Clans are dependent on cousin marriages to maintain the close ties and control of family assets. Clans demand strong loyalty and enforce it with strict honour codes which discourage expressions of individuality and with severe punishments for transgressions. These clans will tend to view outsiders with suspicion, distrust, or even disgust which means outsiders can be exploited mercilessly or even abused. Some people say that we should use the term ‘grooming clans’ rather than ‘grooming gangs’ because the perpetrators of these offences are often members of the same clan, and the clan facilitates a culture of silence about the abuse.
Clans, then, act to prevent assimilation or integration. They encourage elite privileges, with loyalty to the clan far exceeding any loyalty to the host nation or culture. Clans are often linked with corruption and crime with a complete lack of respect for the rule of law. Clannism has economic costs for the host nation, not merely in terms of healthcare, but also in terms of the various social costs to society outlined above. Where clans prevail, there is no prospect of community integration.
Henrich tells a revealing anecdote about an Afghan man being asked why he chose to vote for a man named Sayyaf for in elections in the post-Taliban years. The answer was: “His family have lived here since the days of Dost Mohammad Khan and longer . . . Did you know that my sister’s husband has a cousin who is married to Sayyaf’s sister-in-law? He’s one of our own.” (p409). Tracing the person as a distant relative was what counted. This is how clans think. It is estimated that 46% of marriages in Afghanistan are consanguineous. Akbar Ahmed tells of a Pashtun leader in Pakistan in 1972 explaining: “I have been a Pashtun for six thousand years, a Muslim for thirteen hundred years, and a Pakistani for twenty-five.”[2] Tribal identities are very powerful.
The Mirpuri community from Pakistan first started immigrating to the UK in large numbers with the construction of the Mangla Dam which displaced over 100,000 people many of whom moved to Britian. This community then expanded through ‘marriage migration’ and has grown significantly in towns like Bradford, Birmingham, and Luton. A recent article describes the almost complete lack of integration of this community. A significant factor is the insistence of marriage within the clan such that even after four generations there are very few marriages of Mirpuri’s to non-Mirpuris. Unless something is done, this community will continue to grow as an Islamic clan sub-culture, rather than integrating in any meaningful way.
A recent study showed that the prevalence of consanguinity in a country is a “major predictor of levels of democracy” and an “important determinant” of corruption and factionalism. This is because of the clannism enabled by consanguineous marriage. Banning cousin marriage would act to disrupt and destroy the power of these clans, just as it did in the Middle Ages. This would be of enormous benefit to Western societies, and indeed to societies all over the world.
Arguments against banning cousin marriage
Patrick Nash has helpfully rebutted several key arguments against banning cousin marriage in his recent article on the subject.[3] Banning cousin marriage has been done in several Middle Eastern states, most democratic east Asian nations, and the majority of US states impose restrictions or outright bans on cousin marriage which have stood the test of time and withstood legal challenges. The argument that banning cousin marriage would be a breach of human rights fails because it is already banned in some EHCR member states and recent case law has established that banning consanguineous marriage is likely to be compatible with human rights.
Another argument made is that banning cousin marriage would ban a longstanding custom in some communities. Polygamy is also a longstanding custom in some communities, but it remains illegal in Britain, as does child marriage. We know that this custom is harmful for those that engage in it and for society in general. Therefore, there is a strong reason to outlaw this custom as has been done in other countries.
Others argue that banning cousin marriage is disproportionate. They argue that we wouldn’t ban marriage of older couples because of increased risk of genetic disease, nor would we ban women of childbearing age from drinking alcohol. Such counterexamples are clearly in a different category of proportionality and are quite different from a legal perspective to cousin marriage. The argument for genetic testing of couples rather than banning cousin marriage is also impractical and unfair and would logically need to be applied to all couples planning to marry. This runs dangerously close to government imposed eugenics type restrictions on who can and who cannot have children. There are many wider benefits of banning cousin marriage that do not rest solely on the health risks.
Everyone agrees that banning cousin marriage will benefit the health of future generations in those communities where it has been practiced. Cousin marriage is a largely imported problem which Western countries are entirely entitled to ban for the good of society.
It is also worth noting that studies have shown that U.S. state bans on cousin marriage have had beneficial economic consequences over time for those communities that formerly practised it. Indeed, those states which banned cousin marriage in the late nineteenth century have tended to be the most prosperous states. China banned cousin marriage in 1980 which has caused kinship ties to weaken and helped to boost the economy. Norway moved to ban cousin marriage last year, and Sweden is proposing a ban which will come into effect next year.
The requirement to register marriages
What many commentators fail to recognise is that there is no point banning cousin marriage unless you insist that religious marriages be registered. The problem is that many thousands of Muslims have Islamic marriages that are not registered in law. This means that the women have no legal rights if their husband divorces them, abandons them, or dies. It also means that they can find themselves part of a polygamous marriage with no rights to object. If we ban cousin marriage in law without enforcing registration of marriages this will have no effect on these communities. Indeed, it is likely to only increase the prevalence of unregistered marriages.
I have been arguing for some years that enforced registration of religious marriages would serve to protect Muslim women and prevent polygamy. It would also remove an estimated 95% of the cases heard by sharia courts which are nearly all about women trying to obtain a divorce or trying to obtain some rights after her husband has divorced her.
In 2019, the Council of Europe called on the UK government to make it a legal requirement for Muslim couples to register their marriages. The independent review into the application of sharia law in England and Wales in 2018 also recommended that Muslim marriage be required to be registered. Pakistan mandates registration of marriages, so does Bangladesh, Indonesia, Tunisia and several other countries. In the UK there is already an offence of failing to register a marriage which should be registered. Failing to register a birth or death is also an offence.
In 2022, the Law Commission proposed that it should be a criminal offence for an officiator to mislead a couple about the legal effect of a marriage ceremony or not to disclose that a ceremony will have no legal effect. It also proposed that a marriage is counted as valid if preliminaries are carried out in the presence of a person whom at least one of them believes to be an officiant. This would significantly increase legal protection for women in particular. Unless we insist, as other countries do, that marriages be registered, banning cousin marriage will have no effect.
The case for banning cousin marriage
YouGov polling from earlier this year found that 77% of Britons think first cousin marriage should be illegal, with just 9% disagreeing. Unsurprisingly, amongst Pakistani or Bangladeshi Britons only 47% thought it should be illegal, with 39% disagreeing.
Banning cousin marriage would certainly have improved health outcomes for those communities where it is practised. It would also increase individual freedom by undermining clan loyalty. It would reduce forced marriage, honour violence, and generally lead to clan authority being undermined. Longer term impacts could include a reduction in community corruption and organised crime, including of grooming clans. It would also lead to greater community cohesion and integration of disparate communities.
The case for banning cousin marriage is compelling, with multiple benefits for society. It has been done in many other countries. It is high time it was done here.
References
[1] Statistics and facts here are frequently taken from: Nash, Patrick S (2024), ‘The Case for Banning Cousin Marriage’, Oxford Journal of Law and Religion 13: 98-118. https://doi.org/10.1093/ojlr/rwae014
[2] Akbar Ahmed, The Thistle and the Drone, (Brookings Institution Press, 2013), 21-22.
[3] Nash, Patrick S (2024), ‘The Case for Banning Cousin Marriage’, Oxford Journal of Law and Religion 13: 98-118. https://doi.org/10.1093/ojlr/rwae014
