Likely genetic defect. Check out some stats on the Muslim world:

Certain genetic disorders are common in the Arab world, where inbreeding rates are 25%-60% http://archive.is/nifGB#selection-1915.101-1951.72

Consanguineous marriage has been the culturally preferred form of marriage in most Arab and Middle Eastern countries https://www.ncbi.nlm.nih.gov/pubmed/22317781

Muslim countries have a mean IQ of 84 https://www.questia.com/library/journal/1P3-2018704731/the-comparison-of-mean-iq-in-muslim-and-non-muslim

The risk of having an IQ lower than 70 (criterion for being “retarded”) increases 400% among children from first-cousin marriages https://www.ncbi.nlm.nih.gov/pmc/articles/PMC392897/pdf/pnas00020-0376.pdf