Typical example is Osama bin Ladin, one of 75 children born to his father, a Yemeni, who typically was married to four of his cousins, many of them sisters (!), at a time, and whose parents were also cousins. He is gangly and abnormally tall, deaf, with poor eyesight and very bad kidneys. He has tics, and is nuttier than Aunt Laura's fruitcake. He could walk down a street in Riyadh completely unnoticed.
Osama is also married to various cousins and also has about 75 children. He is no exception in Saudi Arabia, where every other store front is an optometrist shop, a hearing aid store, or a dialysis facility. The gene pool is very dense. This is recognized by the Saudi government, which encourages the importation of brides from elsewhere in the Muslim world, particularly, Egypt,(where they may be just as inbred) Lebanon, and Turkey. Or if one is a really rich Saudi, one buys one's son a Bosnian, or Albanian bride.
Birth defects, both physical and mental are far more common than amongst western populations. Bottom line, we're playing cards with folks who ain't got a full deck, and too many jokers.