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Cousin marriages are encouraged in islam. This leads to genetic defects in offspring and possibly harming their mental development. Obviously that would affect IQ levels.
Muslim marriages are often arranged compared those in the west. So even though the woman in such a marriage might have a high IQ she does not have a say in what genes are past onto her offspring. Natural selection is more impeded.
Lower IQ is like a vicious circle in that it leads to lower employability and lower earnings if they do get a job. So they have less resources to deal with the problems that come from having a lower IQ.
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