Kissing Cousins: How risky is it, really?

by Haifa Haroon on April 13, 2013

Everyone gives out marriage advice. The most memorable ones for me are both from former teachers:

1. Marry someone that can fix it or can afford to pay someone to fix it.

2. Marry someone that is your complete opposite – find a strapping, young Swede.

I’m not sure why the Swedes were singled out but that bit of advice was given by my AP Biology teacher when we were covering the genetics section. Although she was only half serious, she trying to make a point: having children with someone with similar genetic material has risks. So, expand your gene pool. Marry a Swede.

“If you weren’t my cousin…”
(courtesy of Arrested Development)

As you’re probably aware, this is true. Certain populations are more likely to be carriers for a genetic mutation. For instance, people of Eastern European descent are more likely to be carriers of the abnormal cystic fibrosis gene. And, people that are related – take first cousins – are believed to share 12.5% of their genes. So, reproducing with someone in your “group” increases the probability of your child receiving two faulty copies of the gene and as a result having the disease.

There are several studies that support this. Specifically, offspring resulting from consanguineous marriages –  between people that share a common ancestor, like cousins – are at a higher risk of heart disease, cystic fibrosis, childhood morbidity and early mortality.

Courtesy of

Although consanguineous marriage is quite taboo in the US – it’s even illegal to marry your first cousin in 25 states, while in others it’s only allowed if you’re both above a certain age or unable to reproduce – over 10% of people in the world are married to a close relative. That’s almost 700 million people.

Growing up, I remember being appalled when I found out that some of my cousins had parents that were close cousins. I also remember being thankful for being spared.

But, marrying your cousin isn’t appalling to everyone and even with current research on the risks – it’s still practiced. So, is it really that risky?

Cousins George Michael and Maeby Funke
(courtesy of Arrested Development)

To my surprise, this issue is debated.

Some believe that the risks are high while others hold that they have been exaggerated. One of the commonly cited issues with the research is that many of the studies don’t account for maternal age, time between births, socioeconomic status – other factors that are associated with morbidity and mortality in children.

Another thing to consider is that certain populations may have a higher prevalence of faulty genes. For instance, a study concluded that the high rates of children born deaf among the UK Bangladeshi population was due to cousin marriages by comparing the number of children from these couples to the general UK population. However, when the comparison group were deaf Bangladeshi children whose parents were not cousins – the condition was still highly prevalent.

So, what is the risk?

After conducting a literature review of studies published from 1965 – 2000, the National Society of Genetic Counselors (NSGC) concluded that there is an additional 1.7 – 2.8% risk of congenital defects among children of first cousins in comparison to the general population (3% risk). They also found a 4.4% increased risk of death before reproductive age in comparison to the general population. However, the risk of consanguineous marriage would be even higher for people whose parents and grandparents were first cousins as well.

Whether or not this additional risk justifies the ban in several US states is up for debate. Still, it does happen – in the US and all over the world and other than marrying someone that is your complete opposite, the safest option according the NSGC may be consulting a genetic counselor to see what your individual risks are.

(Edited: 4/14/13)

Gaythia Weis April 14, 2013 at 6:39 am

I think that the body of this article is quite good. I would like better concluding paragraph, one that sums up the material on the Bangaladeshi study with the statement: “However, the risk of consanguineous marriage would be even higher for people whose parents and grandparents were first cousins as well.” I believe that the key issue is embedded in a question as to how much inbreeding over how many generations of narrow population base is too much?
I also feel that the introductory paragraph is a distraction that does not lead into the material well. Besides, I worry about your former AP Biology teacher. Didn’t a suggestion, even in jest, to marry a blue eyed blonde, seem a bit racist?

Haifa Haroon April 14, 2013 at 1:41 pm

Hi Gaythia,

That’s an important point. I didn’t run into studies that looked at that specific question. Most of the studies on the health impacts of the offspring of these couples have been in areas where marriages between relatives is a norm – which makes sense but the increase in negative health outcomes may be more due to generations of consanguineous marriages before them rather than the consequence of one time event. As a result, it’s difficult to extrapolate these results to the US population, where it isn’t as common.

Thanks for reading!

Michael April 14, 2013 at 6:18 pm

Have they established the extent to which the body’s own alarm systems can alleviate the risk? I’m thinking about the studies suggesting that the sweetness of a partner’s kiss or (for women on their period) a partner’s smell can be signs of genetic compatibility or lack of same.

Angela April 16, 2013 at 8:35 am

Thank you for the post, Haifa! I find both the scientific and the policy aspect of it very interesting. I came across some studies on island populations and the prevalence of certain diseases or ‘defects’, e.g. on the remote British island Tristan da Cunha . If I remember correctly, they have established their own rules around marriage to ensure a vaguely healthy population.

Thurston Howell, III April 17, 2013 at 10:48 am

No reference to the Habsburg Lip? Leaving disappointed.

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