I had never thought of inbreeding as a possible reason for so many Muslims having mental issues. But, of course, if they continue to only relate to their own, inbreeding is bound to take place. And then it is not that unusual in the ME. From Wikipedia:
'
Cousin marriage is allowed and often encouraged throughout the
Middle East.
[1] Anthropologists have debated the significance of the practice; some view it as the defining feature of the Middle Eastern
kinship system while others note that overall rates of cousin marriage have varied sharply between different Middle Eastern communities.
Biological aspects
Marriage between first cousins doubles the risk of children being born with birth defects, according to a study seeking answers to the higher than expected rates of deaths and congenital abnormalities in babies born from such marriages.
Dr Muhammad Ali, a District Health Officer (DHO) at Charsadda's district hospital, says that first-cousin marriages result in children with genetic disorders. "Birth defects due to inter-family marriages is not just prevalent in Mian Kalay; this can happen anywhere in the world where there is inbreeding.'
NB
Mian Kalay (میاں کلے
is a town of Tehsil Alpurai in Shangla District of Khyber-Pakhtunkhwa province of Pakistan. It is the main village of Pirabd Union Council of Tehsil Alpurai.
