—“Are there any morphological differences between the brain of a highly intelligent person and a person with average intelligence?”—

Three positive factors:

1 – greater neurogenesis

2 – greater neural density

3 – greater white matter (reduced friction)

Three negative factors

4 – Lack of defect in biochemistry (or other illness)

5 – Lack of defect in personality trait (brain structure and chemistry)

6 – Lack of defect due to trauma (of any kind).

And one less obvious:

7 – False knowledge or beliefs (non-correspondence). Certain sets of ideas are incredibly attractive but entirely destructive to our ability to think.

We should note that so far, (as most of us expected)

a) intelligence is influenced by a very large number of genes.

b) unfortunately most influences are negative not positive.

HOWEVER

That means:

c) that potential intelligence does not require we increase any substantial capacity.

d) that potential intelligence can be incrementally increased by cumulative, specific, genetic corrections.

AND

f) Ot seems likely that intelligence then developed a long time ago by accident but through reproduction we have not been able to produce dominance in intelligence without controlled reproduction (like we do with animals),

OR

g) Or the innate possibility was there originally and we have actually devolved from it. This hypothesis isn’t as strange as it originally sounds. Its entirely possible that the rapid increases in our ability to communicate produced greater selection pressure on verbal ability than it did intelligence, and we began to function more as a collective (social) intelligence than individually intelligent agents who imitated each other. The relationship between brain size and intelligence isn’t linear but it exists, and we have smaller (less expensive) brains than both Neanderthals and Cro Magnon’s for example.

In other words, we might have passed peak genetic ability in the past but because of verbal communication reduced the cost and size of our brains, and as such, increased the survival of our weakest.

We don’t know yet.