Cousin marriage

Cousin marriage (sometimes called cousincest) is marriage between cousins. In some places like the Middle East it is common. In other places it is illegal. A famous cousin marriage is that between Charles Darwin and his wife Emma. 
It is believed that the general laws and customs relating to marriage have been influenced by examples of birth defects. Most societies have some kind of limitation to marriages of the type: the closer the genetic relationship, the more likely it is to be banned.
So, the main argument against cousin marriages is that it increases the likelihood of genetic defects occurring in children of such marriages. The classic example of this is the occurrence of haemophilia (and some other conditions) in the royal families of Europe. The repeated marriages between close relatives increased the chance of recessive genes arriving on both chromosomes of a pair, so causing defects to appear. Of course, pre-scientific societies would not have known this. However, they would have seen the results of continual close marriages, such as the brother-sister matings in the Ancient Egyptian royal lines.