Regular readers will remember this topic, but here is the link to a previous post:
Interesting work on the link between cousin marriage and intelligence has been published by Farid and Afzal who have collected Wechsler test scores on inbred children in Muslim areas of India. They chart a very substantial effect of genetically driven cognitive decline, with consequentially higher rates of mental retardation in the inbred families. The differences are enormous: 25 IQ points of retardation on average, and a massive, mind-bending 37 IQ points for the most inbred. Can it really be as bad as that? Has inbreeding caused different groups to diverge so much that they are an average 1.67 standard deviations apart? Think of the heated debates and the social and political ramifications of a 1 standard deviation difference between African Americans and European Americans in the USA. Intellectual differences of close to 2 standard deviations will be of greater consequence in the Muslim world and beyond.
Mohd Fareed and Mohammad Afzal. Estimating the Inbreeding Depression on Cognitive Behavior: A Population Based Study of Child Cohort. Published: October 14, 2014. DOI: 10.1371/journal.pone.0109585
They say: A cohort of 408 children (6 to 15 years of age) was selected from inbred and non-inbred families of five Muslim populations of Jammu region. The Wechsler Intelligence Scales for Children (WISC) was used to measure the verbal IQ (VIQ), performance IQ (PIQ) and full scale IQ (FSIQ). Family pedigrees were drawn to access the family history and children’s inbred status in terms of coefficient of inbreeding (F).
They summarise their results: 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). We observed an increase in the difference in mean values for VIQ, PIQ and FSIQ with the increase of inbreeding coefficient and these were found to be statistically significant (p<0.05). The regression analysis showed a fitness decline (depression) for VIQ (R2 = 0.436), PIQ (R2 = 0.468) and FSIQ (R2 = 0.464) with increasing inbreeding coefficients (p<0.01).
Inbreeding (consanguineous marriages among humans) produces homozygous offspring, since the mating of pairs occurs between genetically closely related individuals. The phenomenon of inbreeding or endogamy, increases the level of homozygotes for autosomal recessive genetic disorders and generally leads to decreased fitness of a population known as inbreeding depression which provides a major focus in clinical studies .
Parental consanguinity has been associated with increased risk of adverse prenatal outcomes including stillbirths, low birth weight, preterm delivery, abortion, infant and child mortality, congenital birth defects, cognitive impairments, malformations and many other complex disorders –.
A study has revealed that the overall incidence of congenital malformations was 2.5 times higher amongst the children of inbred families when compared to that of non-inbred families . 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 .
Genealogical information (family pedigrees) up to five generations back (volunteered by the parents) helped in ascertaining the consanguinity persuasion of their marriage and thus, child’s inbreeding status was determined. The information provided by the parents was cross checked by seeking help from the elder members of the family. In case of ambiguity such data were omitted.
Wright’s path relationship method for calculating the coefficient of inbreeding (F) was used for all mating types following the order: double first cousin (F = 0.125) > first cousin (F = 0.0625) > first cousin once removed (F = 0.03125) > second cousin (F = 0.0156). In non-consanguineous families or for the distant marriages, the coefficient of inbreeding was effectively zero (F = 0.000) .
The authors did full Wechsler tests, so this is a gold-standard assessment, and somewhat unusual in standard publications on intelligence with large sample sizes. In another unusual move in contemporary publications, they have plotted out their data so as to show the distributions. Respect. I am almost reaching out to give them the, as yet mythical, Thompson Prize for Plain Statistics. The intelligence scores are massively shifted to the left (lower intelligence) in the inbred group.
The mean difference in VIQ, PIQ and FSIQ showed a significant inflation with the increase of the degree of inbreeding (p<0.05, using post-hoc tests) and followed the order: double first cousin > first cousin > first cousin once removed > second cousin > non-inbred. This indicates the risk for cognitive impairments or ID was more common among the subjects having greater value of inbreeding coefficients.
The Table below shows the massive drop in Verbal, Performance and Full Scale intelligence associated with consanguinity measured by the F inbreeding coefficient. For the guidance of those about to marry, the worst effects are when two double cousins marry (see list above for verbal descriptions) and check the figures below for the F coefficient categories.
The Full Scale results are drawn out here.
Actually, I may have to hold back the Thompson prize, because they have now reversed the direction of IQ scores, so that high scores are on the left, against usual convention. Pity the poor reader. Nonetheless, the wind blowing through these intellects is clear: the Double First Cousins peak in severe mental handicap, the Non-inbred are at virtually normal scores.
This is all pretty dramatic stuff. The authors have done well. Good sized sample, excellent quality intelligence testing, plainly described results.
However, I am not one to let any researcher get away lightly. The authors do not give the final group IQ, nor do the answer the obvious question: if 45% of this Muslim sample are inbred, what would their IQs look like if they stopped inbreeding?
For general guidance, the overall group IQ would be interesting to compare with other measures of Middle East Muslim intelligence, and with Indian intelligence estimates. Lynn’s mean value for India is about 82, Pakistan 84. Specifically, in this sample it would appear that Muslims who avoid cousin marriage have IQs of 96.5 which is relatively close to Greenwich Mean IQs. Seen in this light, one bad cultural habit has damaged the intellects of normally able people.
I would like to see the sample sizes for each level of inbreeding, so that I can do a few more back-of-the-envelope calculations on what might happen over a few generations if we let loose that great Western invention: Romantic Love. This provides the perfect excuse to ditch your responsibilities to your pushy parents, up sticks and strike out into the blue yonder looking for True Love, far away from your cousins. Time for Bollywood to triumph over tradition.