"The social multiplier posits that other people are the most important feature of our cognitive development and that the mean IQ of our social environs is a potent influence on our own IQ. If this were so, the IQs of adopted children should be associated with the IQs of their adoptive parents, and there should also be a strong correlation between the IQs of unrelated children reared in the same adoptive families."

This is a straw man. I think JR Harris has already demonstrated how little the family environment effects psychological development. The strongest arguments for environmental causes to group differences (as Charles Murray himself says in Commentary) are cultural explanations.

Lynn refuses to address the strongest argument (culture), and tries to prop up his nutrition explanation instead. For those who want to see the nutrition argument thoroughly toppled, check out Flynn's book.