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Archive for the 'consanguinity' Category

The practice of consanguineous marriages in our modern societies

AlexandraBridarolli8 February 2019

This is the fourth and final segment in a series on incest; you can go back and read the previous segments on incest in ancient Egypt, incest in the Hapsburg family, and incest in nature.


As surprising as it may seem, consanguineous marriages are nowadays respected and practised among more than one billion of the world’s population, in particular in the Muslim countries of North Africa, Central and West Asia, and in most parts of South Asia (Jalal Abbasi-Shavazi 2008) with consanguinity rates reaching 20–50% (Hamamy 2016). It has been shown (Bittles 1994, 2001) that sociocultural factors, such as the maintenance of family structure and property, ease of marital arrangements, better relationships with in-laws, and financial advantages relating to dowry, seem to be strong contributory factors in the preference for consanguineous unions.

In countries with civil unrest, consanguineous marriages are preferred because close-kin marriage is regarded as safeguarding for personal and family. It has also been suggested that marriage dissolution and divorce is lower among cousin couples. Studies have indicated that women in first-cousin marriages are protected against intimate partner violence. In another study focussing on cases of consanguinity in Iran (Jalal Abbasi-Shavazi 2008), the authors also demonstrate that in this country the ethnicity, province and area of residence have remained by far the most important determinants of the practice of marriage to biological relatives. In contrast, the modernization variable, education, had no significant effect upon behaviour, the effect being only for those with tertiary education.

Figure 1: Global prevalence of consanguinity (Bittles 2009)


In other countries, the genetic risk associated to consanguineous relations is taken very seriously and all is done to avoid it. This is the case with Iceland. With a population of only 320.00 inhabitants, the risk of incest is really high. De facto, in Iceland everyone shares at least one family relationship. It is said that one of the most asked questions during a first date is: “Hverra manna ert þú?” Which means “ Who are you, who is your family?” To avoid any chance to end up dating your cousin, 3 engineers have recently created a smartphone app called ÍslendigaApp in which people can check the family genealogy and any family relationship with their intended in a few seconds. It even has a “bump” option which gives the info on family relationships when the partners’ phones are clashed and can even send an “incest alert” when the two partners are close family members.

At the origin of this app, there is a genealogy website called Íslendingabók (i.e. Book of Icelanders). Launched in 2010, the database lists the family relationships between Icelanders going back 1200 years. To check the information related to your family, any inhabitant of the Island only needs to provide his/her name and national identity number. Quickly, the website starts to be much more than just a catalogue of genealogy trees and people started to use it to check any possible family connections to their partner.


Here the series on Incest ends. Through the different articles of this series, I have tried to elucidate the incest taboo, find its sources, and understand its origins through different historical cases — Tutankhamun or the Habsburg family — but also through scientific cases.  All these examples have shown us that this taboo seems deeply anchored in nature and culture but that neither nature nor some cultures totally prohibited it. So where to find its source? The questioning hits some dead-end; the answers provided are always partial. Psychology has tried to stick its nose into it while some scientists are waiting for the discovery of a possible “incest” gene. While we wait for answers, I would like to ask you: “Why do you find incest disgusting?” I hope these articles gave you some food for thoughts and starting points for further questioning.

“There is nothing either good or bad, but thinking makes it so” (Hamlet by W. Shakespeare).



Bittles A. H. (1994). ‘The role and significance of consanguinity as a demographic variable.’ Population and Development Review 20(3):561-583.

Bittles A. H. (2001). “A Background summary of consanguineous marriage.” Center for Human Genetics, Edith Cowan University, Perth.

Bittles, A. H. and M. L. Black. “Consanguinity, Human Evolution, and Complex Diseases.” Proceedings of the National Academy of Sciences 107.Suppl 1 (2010): 1779-86. Web.

Hamamy, Hanan, and Alwan, Sura. “Chapter 18 – The Sociodemographic and Economic Correlates of Consanguineous Marriages in Highly Consanguineous Populations.” Genomics and Society, 2016, pp. 335–361.

Jalal Abbasi-Shavazi, Mohammad, et al. (2008). “modernization or cultural maintenance: the practice of consanguineous marriage in Iran.” Journal of Biosocial Science, 40(6):911–933.

Incest in Nature

AlexandraBridarolli20 December 2018

This is the third segment in a series on incest; you can go back and read the previous segments on incest in ancient Egypt and incest in the Hapsburg family.

Firstly, did you know that despite the earliest forms of life emerging around 3.8 billion years ago, sex has only existed for 1.2 billion years? Before that, asexual reproduction was the only form of reproduction to evolve. When you think about it, this is the most extreme type of incest, reproducing yourself with …yourself, cloning yourself. Nowadays, most mammals tend to not engage in inbreeding. If they do, we have seen that incest can lead to depression inbreeding with offspring experiencing health problems. For this reason, scientists used to think that Nature might have weeded out incestuous behaviour through natural selection.

However, recent studies have actually shown that incestuous behaviour has not completely disappeared and that it is more common than generally thought. Some species are asexual or still breed with themselves in situations where there is no advantage to sex; others commit incest where there is no penalty to inbreeding. And guess where those incestuous species are mostly found? Islands and mountaintops. In these isolated places, it is difficult to find someone who does not fit somewhere in your family tree.

Incestuous species

  • Mongoose

Mongoose live in close-knit groups with a median size of 18 adults. Each group has both male and female dominant members, who do most of the breeding and reproducing—those on the periphery only reproduce occasionally. Most group members remain with their group for their entire lives. This close-knit living arrangement has led to a high incidence of incest. A study has found that 64% of newborn pups were the result of mating between members of the same natal group (Nichols, 2014). Father/daughter incest was documented eight times over the course of the study run over 16 years; no mating attempts between mother/son were reported. The researchers point out that females tend to have short lives and generally die before their sons are old enough to mate with them.

Yellow mongoose, Cynictis penicillata


  • Whiptail lizards

This one is with no doubt my favourite.

Some women might have dreamed of a world with no men. Whiptail lizards have done it. Females whiptail lizards are able to clone themselves. And this is not the only species with this capability. There are actually quite a few, 80 groups to be precise, which include amphibians, reptiles, and even fish. But the specificity of these female lizards is that though they don’t need to have sex to survive, they still display mating behaviours, meaning that females sometimes mount other females. Scientists think this behaviour is hormonally driven; high progesterone levels may cause females to mount others. But they probably don’t just bump cloacal regions for fun. Studies have shown that females who are mounted by another female are more fertile than those who go it alone, likely because the mounting behaviour promotes ovulation (Wade, 2013).

Mating behaviour among whiptail lizards: female lizard mounting another female.


  • Spotted salamanders

Among spotted salamanders, DNA analysis shows inbreeding at the level of first cousins, on average. Despite having hundreds of possible mates to choose from, females tended to fertilize their eggs with sperm from related males.

Spotted salamander

Interestingly, in some cases, the natural selection mentioned earlier seems to contradict other studies showing that for some animal or insects, inbreeding within first cousins or brother/sister gives better chance of survival to the offspring. Inbred ambrosia beetles, for example, fared no worse than outbred insects, and the eggs produced by brother-sister pairs are likelier to hatch than the eggs of unrelated pairs (Andersen 2012). Similarly, another study has found that for at least one fish species, fathers from brother-sister couples spent more time, on average, defending their caves and that both parents tended to pay more attention to their kids than unrelated couples.” How to explain this? The ecologist who supervised the study reports, “Couples which are full siblings are more cooperative in brood care. … [T]he males and females stay with the offspring for several weeks and guard them—they defend them—and there’s less aggression between full siblings.”

Stay tuned for next and final segment in a series on incest. We will talking about the practice of incest in modern societies: Modernization or cultural maintenance?


Andersen, H., Jordal, B., Kambestad, M., & Kirkendall, L. (2012). Improbable but true: The invasive inbreeding ambrosia beetle Xylosandrus morigerus has generalist genotypes. Ecology and Evolution, 2(1), 247-257.

Nichols, H., Cant, M., Hoffman, J., & Sanderson, J. (2014). Evidence for frequent incest in a cooperatively breeding mammal. Biology Letters, 10(12), 20140898.

Wade, J., Huang, J., & Crewst, D. (1993). Hormonal Control of Sex Differences in the Brain, Behavior and Accessory Sex Structures of Whiptail Lizards ( Cnemidophorus Species. Journal of Neuroendocrinology, 5(1), 81-93.

The rampant consanguinity in the Spanish branch of the Habsburg family

AlexandraBridarolli18 October 2018

Welcome back to this series of articles on Consanguinity in History. In my previous article, you have heard about incest in Ancient Egypt and the case of Akhenaton and his son Tutankhamun. Let’s continue our investigations around consanguinity and look at another famous case: Charles II from the Spanish branch of the Habsburg family. We often hear that consanguinity is dangerous for the future child, but how dangerous is it exactly and why? The family story of this king followed might give you some food for thoughts.

Between the 15th and the 18th century, the Habsburg family ruled the Holy Roman Empire and, as such, was the most influential and powerful royal family in Europe. In the 16th century, the family separated into the senior Habsburg Spain and the junior Habsburg Monarchy branches, who settled their mutual claims in the Oñate treaty.

The kings of the Spanish Habsburg dynasty, and of the Habsburg house in general, are known to frequently marry close relatives in such a way that uncle-niece, first cousins and other consanguineous unions were prevalent in that dynasty.

Figure 1: Family tree of the Spanish branch of the Habsburg family (kings are in capital letters) showing the inbreeding among Charles II ancestors. (Source: Alvarez et al.)

This branch disappeared in 1700 with Charles II, which many say was because of his family’s rampant inbreeding (see Figure 1). Charles II was indeed famous for being one of the ugliest kings. His nickname was El Hechizado or the Bewitched. He probably suffered from two genetic disorders. First, there was combined pituitary hormone deficiency, a disorder that made him short, impotent, infertile, and weak with a host of digestive problems. The other disorder was distal renal tubular acidosis, a condition marked by blood in the urine, weak muscles and having an abnormally large head compared to the rest of the body.


Figure 2: Portrait of Charles II of Spain (1661-1700) as well as two of his uncles and ancestors (Charles V (1500-1558) and Emperor Leopold I (1640-1705) (credits: Wikimedia Commons).


In order to understand the origin of these disorders, scientists have often used genetic analysis (such as in the case of Tutankhamun and Akhenaten) but not always. Gonzalo Alvarez et al. at the University of Santiago de Compostela recently came up with another innovative approach that enabled them to study 3000 family members of the Spanish branch of the Habsburg family over 16 generations. Using computational calculation of the coefficient of inbreeding (F) of each family member, the team was able to unravel the family history and its consequences on Charles II genetic disorders. The coefficient of Inbreeding (F) corresponds to the probability of finding, at a given position on a chromosome, two genes which are identical by descent. For two first cousins, for example, this probability will be equal to 1/16. For these reasons, consanguinity and inbreeding may significantly impact the occurrence and recurrence of recessive conditions and congenital anomalies (Holt 2013). This may lead to birth defects or children with genetic conditions.

These researchers showed that the inbreeding coefficient for Charles II (0.257), Phillip III (0.218) and prince Charles[1] (i.e. Don Carlos) (0.211) were the highest measured for all the kings of the Spanish Habsburg. This is not surprising as they were all born from either uncle-niece (Charles II and Philip III) or double first cousin (Prince Charles) marriages. However, what was surprising is that the coefficient calculated for each of them were almost twice the expected value for those types of consanguineous marriages (F = 0.125 in either uncle-niece or first cousins relationships) and very close to the expected value in an incestuous union as parent-child or brother-sister (F = 0.250 in both cases). These results were particularly stricking as they showed that “The inbreeding of the Spanish Habsburg kings was not only the consequence of a few generations of unions between close relatives as it is sometimes claimed” but that “ancestral consanguinity from multiple remote ancestors makes a substantial contribution to the inbreeding coefficient of the Spanish Habsburg kings and the contribution of this remote consanguinity is very similar in magnitude to that due to close consanguinity”.

By looking at death records for the family, Alvarez also found that children were much less likely to survive till their tenth birthday if they were born to kings with high F-values. The growing degree of inbreeding in the family meant that fewer and fewer children made it to adulthood, leaving the entire line resting on an infertile, handicapped and short-lived king. Paradoxically, it is thus the same desire that pushed the royal family to preserve the purity of their blood and to keep the “power” within their family that led them to lose it.

But inbreeding did not impact directly the royal lineage. It is, instead, the repeated inbreeding practice that gradually weakens the descendants’ mental and physical health. Incest simply increases the risk that the two parents share the same congenital condition which could then be transmitted to the child. But, what prevents two parents from different families to also share this anomaly? Nothing. Also, it has been shown that having a child with your first cousin raised the risk of a significant birth defect from about 3-to-4 percent to about 4-to-7 percent (Bennett et al., 2002). The authors concluded that this difference wasn’t enough to justify genetic testing of cousin couples and that most of the stigma associated with cousin unions in occidental cultures has little biological basis.

The incest taboo is resolutely very strong…

In the next episode: incest in the animal kingdom. An article in which you will hear about mongoose, termite which reproduce by producing clones of themselves, fish, salamanders and many more. Incest in nature is surprisingly more common than what you would think…


Alvarez G., Ceballos F.C., Quinteiro C. (2009) The role of inbreeding in the extinction of a European royal dynasty. PLoS ONE 4: e5147.

Bennett, R., Motulsky, L., Bittles, A., Hudgins, G., Uhrich, A., Doyle, L., . . . Olson, R. (2002). Genetic Counseling and Screening of Consanguineous Couples and Their Offspring: Recommendations of the National Society of Genetic Counselors. Journal of Genetic Counseling, 11(2), 97-119.

Holt, R. L., and Trepanier, A. (2013). Genetic Counseling and Clinical Risk Assessment-Chapter 21. Emery and Rimoin’s Principles and Practice of Medical Genetics, pp. 1–40.

[1] Prince Charles was also strongly affected by his ancestors’ consanguineous unions. His behaviour suggests that he suffered from some serious mental problems. Rumour in the Spanish court had it that he enjoyed roasting animals alive and in one occasion blinded all horses in the royal stables. At age eleven he ordered the whipping of a serving girl for no known reason. Stories about Charles’ misconducts are numerous.