Incest is a common practice among marriages in Iran, with first cousins marriage estimated to be around 40% of all Iranian marriages. This is due to the fact that many middle eastern families want to keep their wealth with in family lines, preventing "outsiders" from gaining access to it, as well as have no one attempt to change their culture, and reduce divorce rates.
The high rate of incest has caused a high rate of child morality, deafness, blindness, and idiocy, among new generations of newborns, as well as reduced IQ rates for the survivors. Arabs have one of the greatest rates of disorders, with around 66% of them being connected to cousin marriage. Cousin marriage has been tested to indicate that first cousin inbreeding mutiplies the likely hood of carrying a recessive genetic disorder by 13 times as compared to the rest of the population.
Well not related by blood, after a law passed on September 26 2013 by Iran's Guardian Council declared a man is allowed to marry their adopted daughters when they are 13 years or older in "an effort to protect child". Despite this marrying stepchildren is still a forbidden practice.