Royal intermarriage


Royal intermarriage is the practice of members of ruling dynasties marrying into other reigning families. It was more commonly done in the past as part of strategic diplomacy for national interest. Although sometimes enforced by legal requirement on persons of royal birth, more often it has been a matter of political policy or tradition in monarchies.
In Europe, the practice was most prevalent from the medieval era until the outbreak of World War I, but evidence of intermarriage between royal dynasties in other parts of the world can be found as far back as the Late Bronze Age. Monarchs were often in pursuit of national and international aggrandisement on behalf of themselves and their dynasties, thus bonds of kinship tended to promote or restrain aggression. Marriage between dynasties could serve to initiate, reinforce or guarantee peace between nations. Alternatively, kinship by marriage could secure an alliance between two dynasties which sought to reduce the sense of threat from or to initiate aggression against the realm of a third dynasty. It could also enhance the prospect of territorial acquisition for a dynasty by procuring legal claim to a foreign throne, or portions of its realm, through inheritance from an heiress whenever a monarch failed to leave an undisputed male heir.
In parts of Europe, royalty continued to regularly marry into the families of their greatest vassals as late as the 16th century. More recently, they have tended to marry internationally. In other parts of the world royal intermarriage was less prevalent and the number of instances varied over time, depending on the culture and foreign policy of the era.
It was not until the study of genetics began in the early twentieth century that the harm caused by inbreeding was recognized. For modern observers, it is easy to see its relation to royal biological problems, most noticeably in the case of the last Spanish Habsburg monarch, Charles II of Spain, incapable of procreation and suffering from a pronounced underbite - the Habsburg jaw.

By continent/country

While the contemporary Western ideal sees marriage as a unique bond between two people who are in love, families in which heredity is central to power or inheritance have often seen marriage in a different light. There are often political or other non-romantic functions that must be served and the relative wealth and power of the potential spouses may be considered. Marriage for political, economic, or diplomatic reasons, the marriage of state, was a pattern seen for centuries among European rulers.

Africa

At times, marriage between members of the same dynasty has been common in Central Africa.
In West Africa, the sons and daughters of Yoruba kings were traditionally given in marriage to their fellow royals as a matter of dynastic policy. Sometimes these marriages would involve members of other tribes. Erinwinde of Benin, for example, was taken as a wife by the Oba Oranyan of Oyo during his time as governor of Benin. Their son Eweka went on to found the dynasty that ruled the Kingdom of Benin.
Marriages between the Swazi, Zulu and Thembu royal houses of southern Africa are common. For example, the daughter of South African president and Thembu royal Nelson Mandela, Zenani Mandela, married Prince Thumbumuzi Dlamini, a brother of Mswati III, King of Swaziland. Elsewhere in the region, Princess Semane Khama of the Bamangwato tribe of Botswana married Kgosi Lebone Edward Molotlegi of the Bafokeng tribe of South Africa.
Examples of historical, mythical and contemporary royal intermarriages throughout Africa include:

Thailand

The Chakri Dynasty of Thailand has included marriages between royal relatives, but marriages between dynasties and foreigners, including foreign royals, are rare. This is in part due to Section 11 of 1924 Palace Law of Succession which excludes members of the royal family from the line of succession if they marry a non-Thai national.
The late king Bhumibol Adulyadej was a first-cousin once removed of his wife, Sirikit, the two being, respectively, a grandson and a great-granddaughter of Chulalongkorn. Chulalongkorn married a number of his half-sisters, including Savang Vadhana and Sunandha Kumariratana; all shared the same father, Mongkut.

Vietnam

The Lý dynasty which ruled Dai Viet married its princesses off to regional rivals to establish alliances with them. One of these marriages was between a Lý empress regnant and a member of the Chinese Trần clan, which enabled the Trần to then topple the Lý and established their own Trần dynasty.
A Lý princess also married into the Hồ family, which was also of Chinese origin and later established the Hồ dynasty which also took power after having a Tran princess marry one of their members, Hồ Quý Ly.

Cambodia

The Cambodian King Chey Chettha II married the Vietnamese Nguyễn lord Princess Nguyễn Thị Ngọc Vạn, a daughter of Lord Nguyễn Phúc Nguyên, in 1618. In return, the king granted the Vietnamese the right to establish settlements in Mô Xoài, in the region of Prey Nokor—which they colloquially referred to as Sài Gòn, and which later became Ho Chi Minh City.

China

Marriage policy in imperial China differed from dynasty to dynasty. Several dynasties practiced Heqin, which involved marrying off princesses to other royal families.
The Xiongnu practiced marriage alliances with Han dynasty officers and officials who defected to their side. The older sister of the Chanyu was married to the Xiongnu General Zhao Xin, the Marquis of Xi who was serving the Han dynasty. The daughter of the Chanyu was married to the Han Chinese General Li Ling after he surrendered and defected. The Yenisei Kirghiz Khagans claimed descent from Li Ling. Another Han Chinese General who defected to the Xiongnu was Li Guangli who also married a daughter of the Chanyu.
The Xianbei Tuoba royal family of Northern Wei started to arrange for Han Chinese elites to marry daughters of the royal family in the 480s. Some Han Chinese exiled royalty fled from southern China and defected to the Xianbei. Several daughters of the Xianbei Emperor Xiaowen of Northern Wei were married to Han Chinese elites, the Han Chinese Liu Song royal Liu Hui 刘辉, married Princess Lanling 蘭陵公主 of the Northern Wei, Princess Huayang 華陽公主 to Sima Fei 司馬朏, a descendant of Jin dynasty royalty, Princess Jinan 濟南公主 to Lu Daoqian 盧道虔, Princess Nanyang 南阳长公主 to Xiao Baoyin 萧宝夤, a member of Southern Qi royalty. Emperor Xiaozhuang of Northern Wei's sister the Shouyang Princess was wedded to The Liang dynasty ruler Emperor Wu of Liang's son Xiao Zong.
When the Eastern Jin dynasty ended Northern Wei received the Jin prince Sima Chuzhi as a refugee. A Northern Wei Princess married Sima Chuzhi, giving birth to Sima Jinlong. Northern Liang King Juqu Mujian's daughter married Sima Jinlong.
The Rouran Khaganate arranged for one of their princesses, Khagan Yujiulü Anagui's daughter Princess Ruru 蠕蠕公主 to be married to the Han Chinese ruler Gao Huan of the Eastern Wei.
The Kingdom of Gaochang was made out of Han Chinese colonists and ruled by the Han Chinese Qu family which originated from Gansu. Jincheng commandery 金城, district of Yuzhong 榆中 was the home of the Qu Jia. The Qu family was linked by marriage alliances to the Turks, with a Turk being the grandmother of King Qu Boya.
Tang dynasty emperors gave princesses in marriage to rulers of the Uyghur Khaganate to consolidate the special trade and military relationship that developed after the Khaganate supported the Chinese during the An Lushan Rebellion. At least three Tang imperial princesses are known to have married khagans between 758 and 821. These unions temporarily stopped in 788, which is believed in part to be because stability within the Chinese empire meant that they were politically unnecessary; however, threats from Tibet in the west, and a renewed need for Uyghur support, precipitated the marriage of Princess Taihe to Bilge Khagan.
The ethnically Chinese Cao family ruling Guiyi Circuit established marriage alliances with the Uighurs of the Ganzhou Kingdom, with both the Cao rulers marrying Uighur princesses and with Cao princesses marrying Uighur rulers. The Ganzhou Uighur Khagan's daughter was married to Cao Yijin in 916.
The Chinese Cao family ruling Guiyi Circuit established marriage alliances with the Saka Kingdom of Khotan, with both the Cao rulers marrying Khotanese princesses and with Cao princesses marrying Khotanese rulers. A Khotanese princess who was the daughter of the King of Khotan married Cao Yanlu.
The Khitan Liao dynasty arranged for women from the Khitan royal consort Xiao clan to marry members of the Han Chinese Han 韓 clan, which originated in Jizhou 冀州 before being abducted by the Khitan and becoming part of the Han Chinese elite of the Liao.
Han Chinese Geng family intermarried with the Khitan and the Han 韓 clan provided two of their women as wives to Geng Yanyi and the second one was the mother of Geng Zhixin. Empress Rende's sister, a member of the Xiao clan, was the mother of Han Chinese General Geng Yanyi.
Han Durang was the father of Queen dowager of State Chen, who was the wife of General Geng Yanyi and buried with him in his tomb in Zhaoyang in Liaoning. His wife was also known as "Madame Han". The Geng's tomb is located in Liaoning at Guyingzi in Chaoying.
Emperors of the proceeding Song dynasty tended to marry from within their own borders. Tang emperors, mainly took their wives from high-ranking bureaucratic families, but the Song dynasty did not consider rank important when it came to selecting their consorts. It has been estimated that only a quarter of Song consorts were from such families, with the rest being from lower status backgrounds. For example, Liu, consort of Emperor Zhenzong, had been a street performer and consort Miao, wife of Emperor Renzong was the daughter of his own wet nurse.
During the Qing dynasty, emperors chose their consorts primarily from one of the eight Banner families, administrative divisions that divide all native Manchu families. To maintain the ethnic purity of the ruling dynasty, after the Kangxi Period, emperors and princes were forbidden to marry non-Manchu wives. Imperial daughters however were not covered by this ban, and as with their preceding dynasties, were often married to Mongol princes to gain political or military support, especially in the early years of the Qing dynasty; three of the nine daughters of Emperor Nurhaci and twelve of Emperor Hongtaiji's daughters were married to Mongol Princes.
The Manchu Imperial Aisin Gioro clan practiced marriage alliances with Han Chinese Ming Generals and Mongol princes. Aisin Gioro women were married to Han Chinese Generals who defected to the Manchu side during the Manchu conquest of China. The Manchu leader Nurhaci married one of his granddaughters to the Ming General Li Yongfang after he surrendered Fushun in Liaoning to the Manchu in 1618 and a mass marriage of Han Chinese officers and officials to Manchu women numbering 1,000 couples was arranged by Prince Yoto and Hongtaiji in 1632 to promote harmony between the two ethnic groups. Aisin Gioro women were married to the sons of the Han Chinese Generals Sun Sike , Geng Jimao, Shang Kexi, and Wu Sangui.
Nurhaci's son Abatai's daughter was married to Li Yongfang. The offspring of Li received the "Third Class Viscount" title. Li Yongfang was the great great great grandfather of Li Shiyao.
The "Dolo efu" 和碩額駙 rank was given to husbands of Qing princesses. Geng Zhongming, a Han bannerman, was awarded the title of Prince Jingnan, and his son Geng Jingmao managed to have both his sons Geng Jingzhong and Geng Zhaozhong 耿昭忠 become court attendants under the Shunzhi Emperor and married Aisin Gioro women, with Prince Abatai's granddaughter marrying Geng Zhaozhong 耿昭忠 and Haoge's daughter marrying Geng Jingzhong. A daughter of the Manchu Aisin Gioro Prince Yolo was wedded to Geng Juzhong who was another son of Geng Jingmao.
The fourth daughter of Kangxi was wedded to the son of the Han Chinese Sun Sike .

Korea

The Silla Kingdom had a practice that limited the succession to the throne to members of the seonggol, or "sacred bone", rank. To maintain their "sacred bone" rank, members of this caste often intermarried with one another in the same fashion that European royals intermarried to maintain a "pure" royal pedigree.
The Goryeo Dynasty had a history of incestuous marriage within the royal family in its early years, starting from Gwangjong, the fourth king, who married his half-sister :ko:대목왕후|Queen Daemok. To avoid scandals, the female members of the dynasty would be ceremonially adopted by their maternal families after birth. This practice of dynastic incest ended with the overthrow of Queen Heonae, the mother of Mokjong, the seventh king, after she attempted to seize the throne for herself and her illegitimate sons by placing these sons as Mokjong's heir, only to be foiled by a coup masterminded by the Goryeo general Gang Jo.
After the Second Manchu invasion of Korea, Joseon Korea was forced to give several of their royal princesses as concubines to the Qing Manchu regent Prince Dorgon. In 1650, Dorgon married the Korean Princess Uisun. She was a collateral branch of the Korean royal family, and daughter of Yi Gae-yun. Dorgon married two Korean princesses at Lianshan.

Japan

The Japanese may not have seen intermarriage between them and the royal dynasties of the Korean Empire damaging to their prestige either. According to the Shoku Nihongi, an imperially commissioned record of Japanese history completed in 797, Emperor Kanmu who ruled from 781 to 806 was the son of a Korean concubine, Takano no Niigasa, who was descended from King Muryeong of Baekje, one of the Three Kingdoms of Korea.
In 1920, Crown Prince Yi Un of Korea married Princess Masako of Nashimoto and, in May 1931, Yi Geon, grandson of Gojong of Korea, was married to Matsudaira Yosiko, a cousin of Princess Masako. The Japanese saw these marriages as a way to secure their colonial rule of Korea and introduce Japanese blood in to the Korean royal House of Yi.

Europe

Medieval and Early Modern Europe

Careful selection of a spouse was important to maintain the royal status of a family: depending on the law of the land in question, if a prince or king was to marry a commoner who had no royal blood, even if the first-born was acknowledged as a son of a sovereign, he might not be able to claim any of the royal status of his father.
Traditionally, many factors were important in arranging royal marriages. One such factor was the amount of territory that the other royal family governed or controlled. Another, related factor was the stability of the control exerted over that territory: when there was territorial instability in a royal family, other royalty would be less inclined to marry into that family. Another factor was political alliance: marriage was an important way to bind together royal families and their countries during peace and war and could justify many important political decisions.
The increase in royal intermarriage often meant that lands passed into the hands of foreign houses, when the nearest heir was the son of a native dynasty and a foreign royal. The Habsburgs, for example, expanded their influence through arranged marriages and by gaining political privileges in what would become Switzerland, and in the 13th century the house aimed its marriage policy at families in Upper Alsace and Swabia. Given the success of the Habsburgs' territorial acquisition-via-inheritance policy, a motto came to be associated with their dynasty: Bella gerant alii, tu, felix Austria, nube!
and her husband, Francis II of France shortly after his coronation
Monarchs sometimes went to great lengths to prevent this. On her marriage to Louis XIV of France, Maria Theresa, daughter of Philip IV of Spain, was forced to renounce her claim to the Spanish throne. When monarchs or heirs apparent wed other monarchs or heirs, special agreements, sometimes in the form of treaties, were negotiated to determine inheritance rights. The marriage contract of Philip II of Spain and Mary I of England, for example, stipulated that the maternal possessions, as well as Burgundy and the Low Countries, were to pass to any future children of the couple, whereas the remaining paternal possessions would first of all go to Philip's son Don Carlos, from his previous marriage to Maria Manuela of Portugal. If Carlos were to die without any descendants, only then would they pass to the children of his second marriage. On the other hand, the Franco-Scottish treaty that arranged the 1558 marriage of Mary, Queen of Scots and Francis, the son and heir of Henry II of France, had it that if the queen died without descendants, Scotland would fall to the throne of France.
Religion has always been closely tied to European political affairs, and as such it played an important role during marriage negotiations. The 1572 wedding in Paris of the French princess Margaret of Valois to the leader of France's Huguenots, Henry III of Navarre, was ostensibly arranged to effect a rapprochement between the nation's Catholics and Protestants, but proved a ruse for the St. Bartholomew's Day massacre. After the English Reformation, matches between English monarchs and Roman Catholic princesses were often unpopular, especially so when the prospective queen consort was unwilling to convert, or at least practice her faith discreetly. Passage of the Act of Settlement 1701 disinherited any heir to the throne who married a Catholic. Other ruling houses, such as the Romanovs and Habsburgs, have at times also insisted on dynastic marriages only being contracted with people of a certain faith or those willing to convert. When in 1926 Astrid of Sweden married Leopold III of Belgium, it was agreed that her children would be raised as Catholics but she was not required to give up Lutheranism, although she chose to convert in 1930. Some potential matches were abandoned due to irreconcilable religious differences. For example, plans for the marriage of the Catholic Władysław IV Vasa and the Lutheran Elisabeth of Bohemia, Princess Palatine proved unpopular with Poland's largely Catholic nobility and were quietly dropped.
Marriages among ruling dynasties and their subjects have at times been common, with such alliances as that of Edward the Confessor, King of England with Edith of Wessex and Władysław II Jagiełło, King of Poland with Elizabeth Granowska being far from unheard of in medieval Europe. However, as dynasties approached absolutism and sought to preserve loyalty among competing members of the nobility, most eventually distanced themselves from kinship ties to local nobles by marrying abroad. Marriages with subjects brought the king back down to the level of those he ruled, often stimulating the ambition of his consort's family and evoking jealousy—or disdain—from the nobility. The notion that monarchs should marry into the dynasties of other monarchs to end or prevent war was, at first, a policy driven by pragmatism. During the era of absolutism, this practice contributed to the notion that it was socially, as well as politically, disadvantageous for members of ruling families to intermarry with their subjects and pass over the opportunity for marriage into a foreign dynasty.

Ancient Rome

While Roman emperors almost always married wives who were also Roman citizens, the ruling families of the empire's client kingdoms in the Near East and North Africa often contracted marriages with other royal houses to consolidate their position. These marriages were often contracted with the approval, or even at the behest, of the Roman emperors themselves. Rome thought that such marriages promoted stability among their client states and prevented petty local wars that would disturb the Pax Romana. Glaphyra of Cappadocia was known to have contracted three such royal intermarriages: with Juba II&I, King of Numidia and Mauretania, Alexander of Judea and Herod Archelaus, Ethnarch of Samaria.
Other examples from the Ancient Roman era include:
Though some emperors, such as Justin I and Justinian I, took low-born wives, dynastic intermarriages in imperial families were not unusual in the Byzantine Empire. Following the fall of Constantinople in 1204, the ruling families, the Laskarides and then the Palaiologoi, thought it prudent to marry into foreign dynasties. One early example is the marriage of John Doukas Vatatzes with Constance, the daughter of Emperor Frederick II of the Holy Roman Empire to seal their alliance. After establishing an alliance with the Mongols in 1263, Michael VIII Palaiologos married two of his daughters to Mongol khans to cement their agreement: his daughter Euphrosyne Palaiologina was married to Nogai Khan of the Golden Horde, and his daughter Maria Palaiologina, was married to Abaqa Khan of the Ilkhanate. Later in the century, Andronikos II Palaiologos agreed marital alliances with Ghazan of the Ilkhanate and Toqta and Uzbeg of the Golden Horde, which were quickly followed by weddings with his daughters.
The Grand Komnenoi of the Empire of Trebizond were famed for marrying their daughters to their neighbours as acts of diplomacy. Theodora Megale Komnene, daughter of John IV, was married to Uzun Hassan, lord of the Aq Qoyunlu, to seal an alliance between the Empire and the so-called White Sheep. Although the alliance failed to save Trebizond from its eventual defeat, and despite being a devout Christian in a Muslim state, Theodora did manage to exercise a pervasive influence both in the domestic and foreign actions of her husband.
Though usually made to strengthen the position of the empire, there are examples of interdynastic marriages destabilising the emperor's authority. When Emperor Andronikos II Palaiologos married his second wife, Eirene of Montferrat, in 1284 she caused a division in the Empire over her demand that her own sons share in imperial territory with, Michael, his son from his first marriage. She resorted to leaving Constantinople, the capital of the Byzantine Empire, and setting up her own court in the second city of the Empire, Thessalonica.

Post World War I era

In modern times, among European royalty at least, marriages between royal dynasties have become much rarer than they once were. This happens to avoid inbreeding, since many royal families share common ancestors, and therefore share much of the genetic pool. Members of Europe's dynasties increasingly married members of titled noble families, including George VI of the United Kingdom, Prince Henry, Duke of Gloucester, Mary, Princess Royal and Countess of Harewood, Prince Michael of Kent, Charles, Prince of Wales, Baudouin of Belgium, Albert II of Belgium, Prince Amedeo of Belgium, Archduke of Austria-Este, Franz Joseph II, Prince of Liechtenstein, Hans-Adam II of Liechtenstein, Prince Constantin of Liechtenstein, Princess Nora of Liechtenstein, Gustaf VI Adolf of Sweden, Princess Désirée, Baroness Silfverschiöld, Infanta Pilar, Duchess of Badajoz, Infanta Elena, Duchess of Lugo, Princess Marie Adelaide of Luxembourg, Princess Marie Gabriele of Luxembourg, Guillaume, Hereditary Grand Duke of Luxembourg, and Princess Charlotte, Duchess of Valentinois or untitled nobility as Philippe of Belgium and Beatrix of the Netherlands, and very often commoners, as Carl XVI Gustaf of Sweden, Victoria, Crown Princess of Sweden, Harald V of Norway, Haakon, Crown Prince of Norway, Henri of Luxembourg, Felipe VI of Spain, Willem-Alexander of the Netherlands, Frederik, Crown Prince of Denmark, Prince William, Duke of Cambridge and Albert II of Monaco have done.
Among Europe's current kings, queens and heirs apparent, only Elizabeth II of the United Kingdom and Alois, Hereditary Prince of Liechtenstein married members of a foreign dynasty, as did the abdicated Juan Carlos I of Spain.
Members of two reigning houses
Examples of royal intermarriage since 1918 include:
Examples since 1918 include:
Recent examples since 1918 include:
It was believed at that time that the matrimonial alliance between the next Nizam and the deposed Caliph would lead to the emergence of a Muslim ruler who could be acceptable to the world powers in place of the Ottoman Sultans.
As a result of dynastic intra-marriage all of Europe's 10 currently reigning hereditary monarchs since 1939 descend from a common ancestor, John William Friso, Prince of Orange.
MonarchCousinRemovedMost recent common ancestorDeath of MRCAGen. from JWF
Elizabeth II of the United Kingdom-------------------9
Harald V of Norway2ndnoneEdward VII of the United Kingdom6-May-191010
Margrethe II of Denmark3rdnoneChristian IX of Denmark 29-Jan-190610
Carl XVI Gustaf of Sweden3rdnoneVictoria of the United Kingdom22-Jan-190110
Felipe VI of Spain3rdonceVictoria of the United Kingdom22-Jan-190111
Albert II of Belgium3rdnoneChristian IX of Denmark29-Jan-190610
Henri of Luxembourg3rdonceChristian IX of Denmark29-Jan-190610
Willem-Alexander of the Netherlands5thonceFrederick II Eugene, Duke of Württemberg25-Dec-179710
Hans-Adam II of Liechtenstein7thonceJohn William Friso, Prince of Orange14-Jul-171110
Albert II of Monaco7thtwiceJohn William Friso, Prince of Orange14-Jul-171110

Muslim World

Al-Andalus

From the time of the Umayyad conquest of Hispania and throughout the Reconquista, marriage between Spanish and Umayyad royals was not uncommon. Early marriages, such as that of Abd al-Aziz ibn Musa and Egilona at the turn of the 8th century, was thought to help establish the legitimacy of Muslim rule on the Iberian Peninsula. Later instances of intermarriage were often made to seal trade treaties between Christian kings and Muslim caliphs.

Ottoman Empire

The marriages of Ottoman sultans and their sons in the fourteenth and fifteenth centuries tended to be with members of the ruling dynasties of neighbouring powers. With little regard for religion, the sultans contracted marriages with both Christians and Muslims; the purpose of these royal intermarriages were purely tactical. The Christian Byzantines and Serbians, as well as the Muslim beyliks of Germiyan, Saruhan, Karaman and Dulkadir were all potential enemies and marriage was seen as a way of securing alliances with them. Marriage with foreign dynasties seems to have ceased in 1504, with the last marriage of a sultan to a foreign princess being that of Murad II and Mara Branković, daughter of the Serbian ruler Đurađ Branković, in 1435. By this time, the Ottomans had consolidated their power in the area and absorbed or subjugated many of their former rivals, and so marriage alliances were no longer seen as important to their foreign policy.
The Islamic principle of kafa'a discourages the marriages of women to men of differing religion or of inferior status. Neighbouring Muslim powers did not start to give their daughters in marriage to Ottoman princes until the fifteenth century, when they were seen to have grown in importance. This same principle meant that, while Ottoman men were free to marry Christian women, Muslim princesses were prevented from marrying Christian princes.

Post World War I era

There are several modern instances of intermarriage between members of
the royal families and former royal families of Islamic states.
Examples include:
There are also numerous cases of intramarriage between cadet branches within the ruling families from the Arabian Peninsula, including the House of Saud, the House of Al Sabah, the House of Khalifa, the House of Al Thani, and the House of Al Said. Other such examples include Prince Hamzah bin Hussein and Princess Noor bint Asem, Hussein of Jordan and Dina bint Abdul-Hamid, Talal of Jordan and Zein Al-Sharaf Talal, and Ghazi of Iraq and Aliya bint Ali, all from the Hashemite dynasty.

Oceania

Hawaii

Royal incest was extremely common in the Kingdom of Hawaii and its predecessors, despite being rare in other Polynesian societies. Among the aliʻi, the ruling class, marriage between blood relatives of the first degree was believed to produce children with the highest rank under the kapu system, equal to that of the gods. A marriage between brother and sister was considered "the most perfect and revered union". It was believed that the mana of a particular aliʻi could be increased by incestuous unions. According to O. A. Bushnell, "in several accounts about Hawaiians, an ali’i who was the issue of an incestuous marriage was noted for a splendid body and a superior intelligence". Writers have suggested that this preference for brother–sister incest came about as a way to protect the royal bloodline. Notable instances of incestuous relationships among Hawaiian royalty were those between King Kamehameha II and his half-sister Kamāmalu, which was a fully fledged marriage, and between Kamehameha III and his full sister Nahienaena. In the latter case, the siblings had hoped to marry but their union was opposed by Christian missionaries.

Morganatic marriage

At one time, some dynasties adhered strictly to the concept of royal intermarriage. The Habsburgs, Sicilian and Spanish Bourbons and Romanovs, among others, introduced house laws which governed dynastic marriages; it was considered important that dynasts marry social equals, thereby ruling out even the highest-born non-royal nobles. Those dynasts who contracted undesirable marriages often did so morganatically. Generally, this is a marriage between a man of high birth and a woman of lesser status. Usually, neither the bride nor any children of the marriage has a claim on the bridegroom's succession rights, titles, precedence, or entailed property. The children are considered legitimate for all other purposes and the prohibition against bigamy applies.
Examples of morganatic marriages include:
Over time, because of the relatively limited number of potential consorts, the gene pool of many ruling families grew progressively smaller, until all European royalty was related. This also resulted in many being descended from a certain person through many lines of descent, such as the numerous European royalty descended from Queen Victoria of the United Kingdom or King Christian IX of Denmark. The House of Habsburg was infamous for inbreeding, with the Habsburg lip cited as an ill effect, although no genetic evidence has proved the allegation. The closely related houses of Habsburg, Bourbon, Braganza and Wittelsbach also engaged in first-cousin unions frequently and in double-cousin and uncle-niece marriages occasionally.
When Francis II, Holy Roman Emperor married Maria Theresa of Naples and Sicily in 1790, they were double first cousins having the same set of grandparents. Francis became the first Emperor of Austria in 1804 and dissolved the Holy Roman Empire in 1806. All later Emperors of Austria and heads of the House of Habsburg were descendants of this union.
Examples of incestuous marriages and the impact of inbreeding on royal families include: