Cousin Marriages: Risks or Rhetoric
A British MP calls for banning cousin marriages due to the risks of birth defects, but what does the evidence say?
Sinwan Basharat, Canada
In the UK Parliament, a Member of Parliament (MP), Richard Holden caused a stir when he called for marriages between first-cousins to be banned in December 2024. He argued that children born from such unions have higher risks of genetic and developmental disorders. In his statement, Holden remarked, “The science is clear. First-cousin marriages should be banned on the basis of health risk alone.” He especially singled out British Pakistani communities, most of whom are Muslim, where cousin marriages occur commonly.
Are MP Holden’s concerns justified based on science? What does the evidence say? And could the proposed ban, based on the risk assessments, lead towards a slippery slope of government infringing on marriage choices?
Assessing Risks
Risks exist everywhere in life. There are risks with being in a car – which the UN has declared to be one of the most dangerous activities for young people across the world. There are risks of simply walking and falling, something the UK’s NHS estimates affects 1 in 3 older adults, many of whom can experience serious injury. Moreover, a growing volume of research shows the vast, wide-ranging, and potentially debilitating effects of air pollution, plastics, and synthetic chemicals found in household products on birth outcomes
When scientists study health risks, they compare how often a specific health outcome occurs in one group compared to another. Although it sounds straightforward, it requires ensuring both groups are identical in all aspects, except the one factor under study – and outside of an experimental study, that is nearly impossible. Most studies assessing risk factors, especially into a complex outcome like birth defects, rely on observation studies that are prone to a higher level of uncertainty since both groups are not identical and many confounding factors (a hidden connection that can make things appear related when they are not) can influence the results. Estimating risk based on these types of studies have to be understood based on context and the relative effect of risks.
What Are the Risks Associated With Cousin Marriages?
Birth defects or congenital abnormalities are highly diverse and can range substantially in severity and their effects. National estimates from the USA suggest that they occur in three to five percent of all births and many occur without any known genetic or environmental determinants.
Observational studies assessing whether cousin marriages specifically are correlated (remember correlation is not causation) with a range of birth defects, often include a long list of study limitations. Factors such as other behaviours, economic status, demographics, and lifestyle factors can influence the results. Some studies estimate that children born from consanguineous couples may be associated with 0.7% to 3.8% higher risk of congenital abnormalities, which is twice the risk compared to non-related couples. But is that information enough information to act on and establish sweeping bans?
If MP Holden is adamant that for the pursuit of public health, the potentially 4% higher risk of genetic or birth defects that may occur among children born to cousin parents, demands banning such marriages, where else could bans be extended? What about other causes of birth defects? Based on this logic, what else is associated with higher risks and could be targeted?
Age of Parents
The NHS’s own data shows that older mothers (ages 40 years and older) have a 7 times higher risk of children being born with genetic conditions than younger mothers (20 years and younger) in the UK. Expecting parents are often shown a graph displaying the exponentially higher risk of chromosomal abnormalities, conditions such as Down syndrome, associated with mothers aged 35 years or older.
The age of fathers, particularly those above the age of 40, is also increasingly being shown in research studies to be associated with higher risks, sometimes 4 to 6 times higher risk of birth defects, mental health disorders, and autism.
Should the UK government also begin enforcing what age people can become parents? Beyond that, for the so-called pursuit of public health, will the government put a limit on what age a person can get married?
Brave New World
Even if cousin marriages are associated with a marginal level of increased risk for birth defects, the very fact of being related does not cause birth defects. Correlation is not causation – parents being related to each other does not mechanistically lead to birth defects. Most of the risk stems from the possibility that offspring may inherit an autosomal recessive disorder, where both parents may be carriers of a genetic mutation. Moreover, these types of genetic conditions are not limited to cousins, but rather could occur among any 2 people in the world.
Does the government envision mandating that all hopeful parents be required to have genetic tests and be approved to be parents only if they are genetically a good match? Will couples be required to have genetic tests prior to getting marriage licences? Is this the dystopian public health MP Holden envisions for the UK?
Efforts to Reduce Birth Defects Already Have a Prime Focus
If MP Holden or other government officials are truly dedicated to helping reduce birth defects, there is an alternative area to focus on that is more likely to reduce birth defects among children. He can help raise awareness and address a severe public health crisis growing in the UK. The UK has one of the world’s highest rates of alcohol use during pregnancy. Estimates suggest that nearly half of pregnant mothers in the UK consume alcohol. During this critical window of life, alcohol exposure to a growing fetus is the leading cause of birth defects, associated with poor birth outcomes, neurological disorders, and developmental delays. This association is not just a correlation, but rather driven from specific mechanisms where alcohol exposure adversely affects the biology of a growing fetus.
The NHS, several UK government departments, doctors, midwives, and other healthcare professionals have raised alarms about the devastating impact and the need for approaches to better support families and reduce preventable harms due to alcohol exposure.
Cousin Marriages Ban is Unreasonable and Lacks Rationality
Cousin marriages have existed throughout history in the UK (the Royal Family no less) and throughout the world. Across many different religions, including Islam, marriages between cousins are permitted and occur, provided there is interest and consent among the couple. In a free society, particularly one that is referred to as ‘modern British society’, is it really the role of the state to be involved in who people should marry and beyond that, who should be allowed to have children? Are cousin marriages a threat to ‘women’s freedom’, as MP Holden remarked, or the proposed policing by the state?
The evidence shows that the potential risks of birth defects from cousin parents can be less than other risk factors (such as age of parents). Even so, such correlated risk factors are magnitudes less in comparison to the causative and extremely damaging effects of alcohol to children in-utero – the leading cause of birth defects. In short, the proposed marriage ban has little justification based on science and it raises ethical and moral dilemmas about the role of the government on private matters of families and couples.
About the Author: Sinwan Basharat is a researcher with a background in molecular genetics and epidemiology. He works for a research agency in Ottawa, Canada. He also serves as a Deputy Editor for the Science Section at The Review of Religions.
