This is a touchy subject for obvious reasons. Nevertheless, there are studies documenting the debilitating impact of inbreeding on intelligence of consanguineous groups, such as this one from India:
“According to the latest statistics of the Palestinians Central Bureau of Statistics, consanguinity is generally high among Palestinians. In general, around 50% of total marriages are consanguineous with little variation between the West Bank and Gaza strip, with 43.4% and 49.3%, respectively. There has been no significant decline in this rate between the years 2000 and 2006 in spite of the public education and media campaign to discourage this kind of marriage.”
Ahmad S. Teebi: Genetic Disorders Among Arab Populations. Springer Science & Business Media.