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IQ and first cousin marriage https://www.sciencedirect.com/science/article/pii/S0160289608001608

Actually, the study referenced the children of double first cousins. There is a big difference, between first cousins who aren't inbred marrying and those who are inbred marrying:

"The study of Bashi (1977) revealed that the extent to which consanguinity affects IQ is proportional to the degree of inbreeding. He found that based on the outcomes of three tests of cognitive ability, the children of double first-cousins (within an Arab population) exhibited on average significantly greater inbreeding depression of test scores than the children of first-cousins, who in turn scored lower than the children of non-consanguineous parents. Bashi also noted that the children of double first-cousins exhibited larger variance in test scores than the children of first-cousins. These findings tend to disconfirm environmentalist theories such as those of Kamin (1980), who proposed that socioeconomic status is the dominant factor in determining the IQ of the offspring of consanguineous mating."