Is It Bad To Have Sex With Your Cousin?

Is It Bad To Have Sex With Your Cousin?

Marrying your cousin might sound icky, but it’s perfectly legal in many countries, including Australia and New Zealand. According to a new large-scale study, the risk of siring offspring with birth defects is actually relatively small; around the same as all expectant mothers over 34.

Six finger picture from Shutterstock

In the largest study of its kind, researchers in the UK analysed data collected from almost 14,000 babies born in Bradford, England between 2007 and 2011 and compared the number of congenital anomalies from both consanguineous (blood-related) and non-consanguineous relationships.

The report found that cousin marriages pose an increase in the risk of birth defects. However, the researchers also noted that the absolute risk of birth defects still remained low.

“This study confirms what has previously been known, that babies born to couples that are related, definitely leads to increased rates of babies born with health problems,” commented Professor Andrew Shelling, Faculty of Medical and Health Sciences at the University of Auckland.

“[However] the increased rate of problems is relatively small, and in cousins, it is not much different to those babies born to unrelated individuals. In an interesting twist, this study shows that the risk is about the same as older women (defined as having babies over 34) having babies.”

Professor Hamish Spencer, Director of the Allan Wilson Centre for Molecular Ecology and Evolution at the University of Otago also released a statement defending consanguineous marriage:

The risk of birth defects in children whose parents are first cousins is often exaggerated in the public’s mind. Most such babies will be fine. All the same, there is an increased occurrence of birth defects in such babies…I would say that this increase can sound large (“twice the risk”) or small (“an increase of only about 3%”) depending on your viewpoint.

In other words, if your cousin is hot, perhaps the risk is worthwhile.

Risk factors for congenital anomaly in a multiethnic birth cohort: an analysis of the Born in Bradford study [The Lancet]


  • The risk of birth defects might not be prohibitive, but the risk of cousin x cousin mating would absolutely increase the compounded likelihood of the offspring inheriting genetic defects and gene mutations. As it stands, one in 25 people carry the cystic fibrosis gene; if you have the gene and you have a baby with someone else who happens to have the gene, there’s a one in four chance your child will have cystic fibrosis. Mate with a first cousin and that obviously poses a far greater risk than mating with someone who is not a blood relative.

    • That isn’t how genetics works, birth defects and genetic disorders are completely different, and it is the same chance of passing of cystic fibrosis no matter the relation you share with the partner.

      • Except that in this example, if you’re in the same family and you have the cystic fibrosis defect in your DNA, then it’s likely your cousin does too. *That’s* what Astrid means by there being an increased risk in making babies with said cousin.

        • Except she specifically stated the chance of having a CF baby with a stranger then said there is a higher risk with a relative, in fact said the chances are “far greater”. It is not really likely your cousin will have the gene, not impossible, but not likely.

      • @piat Sorry mate you aren’t being clear and confusing people. Congenital (birth) defects can be related to genetic abnormalities but not always. The chance of passing on a genetic abnormality, such as cystic fibrosis, does matter if your partner is in a high risk category (carrier or has the disease – which is 1 in 25 in people with European decent).

        • You are correct, but that still has no bearing on relation to the partner, cousin or not it is still the same risk if they are high or low risk.

          • Cystic fibrosis is an autosomal recessive genetic disorder. Autosomal recessive genetic disorders are more likely to occur if two parents are related because they are more likely to have one copy of the same altered gene. My wife and I were counselled extensively about this subject due to her carrying the gene, although we aren’t related. The risk factor, I’m told, is only slightly higher with blood relatives but it is still higher than that of non-blood relatives.

          • So basically you have the same risk as if you were to shack up with someone of european decent…

            So perhaps they should start saying “having sex with a europeans is like rooting your cousin” . lol
            So in other words it seems like it’s not worth worrying about as you probably wouldn’t have a problem rooting a european… 🙂

  • After learning that a family member married his first cousin, I looked into what Aus law was around family relations and marriage.. I’d always assumed that it was illegal to marry a first cousin (probably as a result of American movies and TV), but not so in Australia. It actually legal to marry anyone who isn’t a direct ancestor or descendant – so you can legally marry your Aunt or Uncle too.

    • Yea, when my wife (not cousin) and I sat down with our celebrant to plan our ceremony.

      She pointed this out, the rule is, you can marry diagonally on the family tree, but not up, down or sideways.

      Weird I know, but apparently it happens.

    • What if your cousin was Scarlett Johanssen and she was just so so so attracted to you…

      Would you be able to live with the shame? O_O

      9/10 would say hell yes.

      • all my mates continuously tell me my cousin is attractive. Whilst no SJ she does frequently enter (and occasionally win) swimsuit and modelling contests. I’ve never had any desire to ‘fool’ around with her.

        Your 9/10 assumption is flawed – when you grow up with someone the attraction is diminished when compared to a movie star you will never see.

  • So anyone notice that the picture of the hand has 5 fingers and a thumb?

  • Misleading headline. The result of the study is that it’s potentially not a good idea making babies with your cousin, and especially if s/he might be over 34 to boot. But go ahead, have sex if you want. Just don’t make babies.

  • Would like to know who submitted this as a question so we can confirm whether it was a Queenslander, a Taswegian or a visiting New Zealander.

    If there’s a follow-up question about sheep, we’ll know for sure.

  • I call Naomi Campbell a “cousin”. Same with Neve Campbell. While I doubt I will ever have sex with either of them, it would be rude to outright reject them if they asked politely.
    [Need to look up some more, famous, Campbell cousins]

  • I married my cousin. After researching Alan H Bittles among others, we determined that ignorance rules in regard to cousin marriage. There are actual scientific advantages as well marrying and bearing offspring with cousins. There is a lower rate of bodily rejection and an increase of purged genetic abnormalities. We are WASP Americans of Irish descent. We didn’t experience the Westermark effect (didn’t grow up around each other) but did have a strong GSA. 20% of all marriages in the world are of first cousins. Einsteins parents were first cousins as were he and his wife. Darwin married his first cousin…
    The West has this ignorant outlook that stems from faulty eugenics study and also to keep families from maintaining power and wealth by manintaining assets within their clan unlike the British and other related european power families. The stigma was attached to the rural families because they had little access to healthcare, flourine (advanced water treatment) especially before the invention of the automobile.
    If you peel the onion back you will find there is no real reason to ostracize cousin marriage. It is sad that we are compared to gay marriage which has nothing at all to do with hetero consanguineous relationships from second degree and beyond relationships. It is theorized that 80% of all marriages from the beginning of the practice have been first cousins. 200,000 first cousins wed in the US annually. Repeated first cousin marriage over multiple generations (double first cousins) are another story.
    The US is archaic in practice and thought when it comes to us.

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