I wonder what are the documented defects?
Oh look.. Wikipedia..
Inbreeding in humans
The taboo of incest has been discussed by many social scientists. Anthropologists attest that it exists in most cultures. As inbreeding within the first generation often produces expression of recessive traits, the prohibition has been discussed as a possible functional response to the requirement of culling those born deformed, or with undesirable traits.[citation needed] Some anthropologists like Charles Davenport advocated the traditional forms of assortative breeding to form better human stock.
Ancient Egypt
Some Egyptian Pharaohs married their sisters; in such cases we find a special combination between endogamy and polygamy. Normally the son of the old ruler and the ruler's oldest (half-)sister became the new ruler. Cleopatra VII and Ptolemy XIII, married and named co-rulers of ancient Egypt following their father's death, were brother and sister. Not only this, but all rulers of the Ptolemaic dynasty from Ptolemy II on engaged in inbreeding among brothers and sisters, so as to keep the Ptolemaic blood "pure".
Royalty and nobility
The royal and noble families of Europe have close blood ties which are strengthened by royal intermarriage; the most discussed instances of interbreeding relate to European monarchies. Examples abound in every royal family; in particular, the ruling dynasties of Spain and Portugal were in the past very inbred. Several Habsburgs, Bourbons and Wittelsbachs married aunts, uncles, nieces and nephews. Even in the British royal family, which is very moderate in comparison, there has scarcely been a monarch in 300 years who has not married a (near or distant) relative.
Indeed, Queen Elizabeth II and her husband Prince Philip, Duke of Edinburgh are second cousins once removed, both being descended from King Christian IX of Denmark. They are also third cousins as great-great-grandchildren of Queen Victoria of the United Kingdom. European monarchies did avoid brother-sister marriages, though Jean V of Armagnac was an exception.
When your family tree doesn't fork.. is always the jest of low income... but apparently the reverse is more the truth.