First Cousin Marriage Ban: Eugenic, Ineffective
A bill that proposes to ban first-cousin marriage in the UK will receive its second reading in the House of Commons on March 7.
The bill, proposed by Conservative former minister Richard Holden, follows the introduction of a ban on cousin marriages that came into effect in Norway in 2023 and a planned ban in Sweden from mid-2026.
Different reasons might be given for proposing to ban first-cousin marriage. However, one significant reason given by supporters of these bans is concern for public health. Holden claimed in his speech to parliament that: “First-cousin marriage should be banned on the basis of health risk alone.”
In the UK, a long-standing research study of childhood outcomes in Bradford, where there has traditionally been a high rate of cousin marriages within the Pakistani community, recently found that children of first cousin parents had higher rates of learning and speech problems and more visits to hospitals and doctors.
The increased incidence of certain genetic illnesses in children of related parents has long been recognised. When parents are closely related, they are more likely to carry the same faulty genes.
If both parents pass on the same faulty gene to their child, the child has a higher chance of developing a genetic illness (about double the risk of parents who aren’t related). The Bradford study had earlier found that first-cousin marriages were linked to 30% of cases of birth defects in the studied population.
The recent study suggests that even once you exclude those children diagnosed with recessive genetic conditions – and even after adjusting for other risk factors such as poverty – the children had higher rates of illness and developmental problems.
Although it is laudable to wish to seek measures to prevent health and learning problems in future children, there is a fundamental ethical challenge.
Banning first-cousin marriage will not prevent children from having genetic illness or health problems, rather, it will prevent some children from being born and mean that different children (with a lower chance of genetic or other problems) are born instead.
A basic legal and ethical principle , defended by the 19th-century philosopher John Stuart Mill, is that states are only justified in restricting the basic freedoms of individuals to prevent harm to others. But if we take the “harm principle” seriously, then the health case for a marriage ban dissolves. There will be no child who is saved from illness or harm because of a law banning first-cousin marriage.
It might be thought that a ban would still be justified, based on community health rather than for the sake of specific children. The idea would be that it would be important to prevent first-cousin marriage because of the high rate of genetic illness in offspring. Perhaps the hope would be to reduce pressure on the health system . But there are several problems with this argument.
First, most children of parents who are first cousins are healthy. The rate of genetic or congenital problems is 6% (compared with 3% in parents who are not related). This means that 94% of children will not have genetic or congenital problems. Or to put it another way, given the small additional risk, over 30 couples would have to be prevented from marrying to prevent one child from being born with an inherited genetic problem. The same argument applies to the extra learning problems seen in the Bradford study that were not diagnosed as genetic problems: most children of first-cousin parents did not have learning difficulties or serious illness.
Next, a ban on cousin marriage to reduce the rates of illness or learning problems in their offspring would represent an attempt to prevent certain people from having children for the sake of benefiting the population. But once we frame it in that way, it is clear that such an effort would be eugenic , based on a particular group’s perceived genetic fitness to reproduce.
Such a policy would be an example of some of the most troubling forms of eugenics: restricting basic freedoms (the freedom to marry and have children) for the sake of the common good.
Third, the health-based reason to ban first-cousin marriages is because of the elevated rate of birth defects and health problems in children. However, the rate of these problems is also increased in parents who are related more distantly. And in close-knit ethnic groups there can be shared genes and increased rate of congenital problems (so-called endogamy ), even without cousin marriage.
If we ban first-cousin marriages, families could shift to others within their extended family. Or, if we wanted to prevent higher rates of birth defects, we might need to ban not just first- and second-cousin marriages, but also marriage within ethnic communities. But that would look even more problematic.
How should we respond then to the high rates of health and learning problems in communities like those in Bradford?
One important response is to be aware of the additional needs of those communities (Bradford has areas that are among the most deprived in the UK ) and to ensure that the needs of children are addressed.
A second response is to provide education to families and to young people who are potentially marrying so that they are aware of the increased risks associated with cousin marriage and can make informed decisions.
Finally, there are more sophisticated and targeted ways of identifying risks for couples while respecting their reproductive rights. So-called expanded reproductive carrier screening could identify before they become pregnant, whether both partners in a couple are carriers for the same genetic illness. That could help them to decide whether to have children together, whether to use other techniques – such as IVF – to prevent genetic illness or to adopt. That expanded screening isn’t currently available on the NHS, but it could be made available to couples who are related.
We should be concerned about higher rates of illness in the children of parents who are related. But the ethical answer isn’t to ban them from getting married.
Dominic Wilkinson receives funding from the Wellcome Trust.