Ad hominem may as well be Latin for "I choose to forfeit the debate on account of disrespect," for in practice it is the choice, conscious or careless, to sidestep an opposing argument and instead attack the character of its advocate. The worst offense of this kind is establishing intelligence as a condition for agreement.
John Derbyshire of National Review quotes a friend of his on interlocution: "I don't find myself in long conversations with people whose IQs are [between 100 and 119], let alone any lower. In software development projects us smarter team members end up having rapid fire complex conversations and at the end explain the conclusions to the lesser minds." This supports the observation of another friend, which is that communication between high intelligence and low intelligence "quickly becomes," as it is more pronounced, "impossible." Isn't that a tidy theory? And it is a theory. Tests conducted during my childhood suggest that I am within the 99th IQ percentile. A bachelor's degree was sufficient for my work and I do not intend to return to university study — yet after half a decade living among the medial, I can't recall a communication failure caused solely or even primarily by a difference in intelligence.
Now, I concede the fact that we can't befriend everybody. Education and avocation attract and repel people, there are things which can be shared only between those who have like minds, and obviously someone with a robust intellect will be capable of learning concepts more quickly and in greater number than someone with less than that. But imputing an interpersonal divide to the perceived inferiority of another presumes a surpassing importance of one's own work and interests — a conceit which is, fitting nicely with Derbyshire's subsequent brief on "zones of commitment," the prepossession of academics.