In-breeding appealed to political and economic dynasts, such as the Habsburgs and Rothschilds, because it doesn't dissipate family assets to too many heirs.
The Habsburg dynasty that reigned over much of Europe from the late medieval period to the last days of WWI is notorious today for the inbreeding that beset Charles II, the Habsburg king of Spain from 1665-1700. Wikipedia explains:
Charles was born in Madrid, the only surviving son of his predecessor, King Philip IV of Spain and his second Queen (and niece), Mariana of Austria, another Habsburg. His birth was greeted with joy by the Spanish, who feared the disputed succession which could have ensued if Philip IV had left no male heir.
17th century European noble culture commonly matched cousin to first cousin and uncle to niece, to preserve a prosperous family's properties. Charles's own immediate pedigree was exceptionally populated with nieces giving birth to children of their uncles: Charles's mother was a niece of Charles's father, being a daughter of Maria Anna of Spain (1606–46) and Emperor Ferdinand III. Thus, Empress Maria Anna was simultaneously his aunt and grandmother and Margarita of Austria was both his grandmother and great-grandmother. This inbreeding had given many in the family hereditary weaknesses. That Habsburg generation was more prone to still-births than were peasants in Spanish villages.
There was also insanity in Charles's family; his great-great-great(-great-great, depending along which lineage one counts) grandmother, Joanna of Castile ("Joanna the Mad"; however, the degree to which her "madness" was induced by circumstances of her confinement and political intrigues targeting her is debated), mother of the Spanish King Charles I (who was also Holy Roman Emperor Charles V) became insane early in life. Joanna was two of Charles' 16 great-great-great-grandmothers, six of his 32 great-great-great-great-grandmothers, and six of his 64 great-great-great-great-great-grandmothers.
Dating to approximately the year 1550, outbreeding in Charles II's lineage had ceased (see also pedigree collapse). From then on, all his ancestors were in one way or another descendants of Joanna the Mad and Philip I of Castile, and among these just the royal houses of Spain, Austria and Bavaria. Charles II's genome was actually more homozygous than that of an average child whose parents are siblings. He was born physically and mentally disabled, and disfigured. Possibly through affliction with mandibular prognathism, he was unable to chew. His tongue was so large that his speech could barely be understood, and he frequently drooled. It has been suggested that he suffered from the endocrine disease acromegaly, or his inbred lineage may have led to a combination of rare genetic disorders such as combined pituitary hormone deficiency and distal renal tubular acidosis.
Consequently, Charles II is known in Spanish history as El Hechizado ("The Hexed") from the popular belief – to which Charles himself subscribed – that his physical and mental disabilities were caused by "sorcery." The king went so far as to be exorcised.Charles II died without issue at age 38, which set off a crisis in Europe's balance of power. His will named as king of Spain a relative who was also the grandson of King Louis XIV of France. Britain objected to the union of France and Spain under the Bourbons. In the ensuing War of the Spanish Succession, John Churchill became Duke of Marlborough for winning the Battle of Blenheim. (His descendant Winston Churchill wrote a six volume biography of his ancestor.)
So, the Habsburgs were genetically doomed forever by inbreeding, right?
Well, on July 4, 2011 died Franz Joseph Otto Robert Maria Anton Karl Max Heinrich Sixtus Xavier Felix Renatus Ludwig Gaetan Pius Ignatius von Habsburg , crown prince of the Austro-Hungarian Empire from 1916-18, who lived a model of a healthy, useful life, died at age 98. He stood against Hitler and Stalin, turned down the throne of Spain and recommended Juan Carlos instead, and served in the European Union parliament for decades. Otto von Habsburg was, by political inclination, ancestry, and family trade, a pan-Europeanist. His ancestor Charles V had ruled over more of Europe (and ruled rather conscientiously) than any man between Charlemagne and Napoleon.
Otto von Habsburg's last great contribution to European unity was cosponsoring the Pan-European Picnic on August 19, 1989 on the Austrian-Hungarian border, where the Soviet Empire sprang a terminal leak. By pre-arrangement with Hungarian authorities, the border gate in what we call "the Berlin Wall" (but which was actually 1800 miles long, running from the Baltic to the Aegean) was opened for three hours during Otto's picnic. Hundreds of East German tourists left the Warsaw Pact countries to join relatives in West Germany.
A few weeks after this genial occasion, the Hungarians decided to make it permanent and stopped stopping East Germans tourists from leaving Hungary for the West. Because there was no serious border control within the Warsaw Pact, a leak anywhere could eventually drain the Soviet Empire of its most valuable inmates. Eventually, the East Berlin authorities gave in on November 9, 1989 and told the wall guards to stop guarding.
Archduke von Habsburg was also a pundit whom I regularly read forty years ago. Charles A. Coulombe writes in Taki's Magazine in Death of an Imperial Pen Pal:
The San Fernando Valley in the 1970s was a very dull place. Hot and dusty, filled with lackluster architectural construction thrown together during the postwar housing boom, it was the last place I wanted to be.
Back in those far-off days, the LA Archdiocese’s paper, The Tidings, ran a column by the Archduke Otto von Habsburg, son of Austria-Hungary’s last Emperor-King.I read him, too. The Tidings' other columnist back when I was 12 was the almost as cosmopolitan Erik von Kuehnelt-Leddihn. We got a quality dose of high-brow Mittel-Europa punditry in the San Fernando Valley
The solution to the genetic woes of inbreeding is to stop inbreeding. Even a modest level of non-inbreeding quickly solves problems like sterility.
From Catholic News:
Otto, who stopped appearing in public after the death of his wife, Regina, last year, is survived by his younger brother, Felix, as well as 7 children, 22 grandchildren and 2 great-grandchildren.You can do some interesting calculations about average fertility per generation using the last paragraph in the obituaries of prominent people (although one caveat is that the obituaries give survivors, not total descendants). It would be interesting to build a model to predict the number of surviving descendants by generation of, say, people important enough to get their obituaries in the New York Times. Use as factors: date of birth, age at death, sex, career, number of marriages, etc.
Take the Archduke as an example. So, among his survivors, Otto had 7 children and an average of 3.14 surviving grandchildren per surviving child. But his 22 grandchildren have only 2 surviving great-grandchildren, so far, or 0.09 on average.
Talk about pedigree collapse.