The rampant consanguinity in the Spanish branch of the Habsburg family

By Alexandra Bridarolli, on 18 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 and final 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.