In some Muslim countries, over 50% of marriages are arranged, and involve cousins:
We found significant decline in child cognitive abilities due to inbreeding and high frequency of mental retardation among offspring from inbred families. The mean differences (95% C.I.) were reported for the VIQ, being −22.00 (−24.82, −19.17), PIQ −26.92 (−29.96, −23.87) and FSIQ −24.47 (−27.35, −21.59) for inbred as compared to non-inbred children (p>0.001). The higher risk of being mentally retarded was found to be more obvious among inbred categories corresponding to the degree of inbreeding and the same accounts least for non-inbred children (p<0.0001).
The risks of inbreeding have long been known:
Inbreeding (consanguineous marriages among humans) produces homozygous offspring, since the mating of pairs occurs between genetically closely related individuals.... Consanguinity has been associated with significant decline in mean values for height, weight and body mass index (BMI) and the subsequent depression on children growth, much influenced in proportion to their inbreeding coefficients with least variation for non-genetic factors . Inbreeding is also thought to predispose offspring to neuropsychological disorders such as hereditary Parkinsonism . Several studies have identified parental consanguinity as an important risk factor for mental retardation or ID , , , and it has been shown that inbreeding is correlated with reduced cognitive performance .
The study results, in graphical terms:
The X axis is Full-Scale IQ. IQ scores of 75 and below are considered mentally disabled in Western countries, anything under 60 is profoundly disabled, generally requiring special education and lifelong assistance. (Which, of course, most developing countries can't afford.) There may be some confounding factors here, such as that higher-IQ, better-educated Muslims may be less traditional and more likely to avoid arranged cousin marriages, and higher-IQ wealthier Muslims may face less risk from malnutrition or pollution. But still, the well-known genetic danger of consanguinity is certainly explaining a lot of this unbelievably huge variance.
This is why IQ is so revealing. The debate on how to develop countries goes 'round and 'round, starting again every time the latest intervention (education, clean water, infrastructure) delivers less-than-overwhelming results. And the blame game goes back and forth, ad nauseam. But this study, and many more like it, point to an obvious factor which few squeamish PC Westerners are willing to discuss openly. Cousin marriage creates generation after generation of people with severe intellectual deficits. Eliminate it, and (after a few generations) you will unleash massive amounts of human capital that has been destroyed for centuries by the dead hand of tradition.