Interesting discussion going on here. One thing I want to add that might serve as a confounding variable of intelligence and genetic disorders is parental age a

Interesting discussion going on here. One thing I want to add that might serve as a confounding variable of intelligence and genetic disorders is parental age at birth. It has extensively been shown that especially the fathers age is linked to the chance of denovo genetic diseases in his children, due to mutations occuring in the ongoing cell replications. One the other hand older parents tend to have children that are better at school (on average), are more successful in their career etc. and therefore might be more likely to take an IQ test and join Mensa.