UCLA researchers have discovered that intellectual performance is strongly correlated with the quality of the mieline covering of neuronal axons in the brain. Which in turn is strongly dependent on genetics.
For starters, mieline is the isolation layer around the "cables" (axons) in our brain and nerves. While neurons have many sinapsis (connections with other nearby neurones), they have one or two longer extensions called axons, which are the "cables" of our nervous system, brain included.
When axons' mieline is thick and therefore provides a good quality isolation, the neuronal signal is stronger and clearer. Instead, when mieline is thinner, the electrical isolation is worse and the neuronal signal weaker. The first case provides for good intelligence, while the latter does not; additionally bad mieline covering is also related to mental illnesses like autism and alzheimer. Mieline thickness also shows a U-shaped pattern in life that is optimized for middle age (the acmé of classical Greeks, located around the age of 40), what explains increase and decrease of intellectual prowess with age.
The UCLA team compared pairs identical twins (natural "clones") and normal syblings, finding more marked differences in mieline quality among the latter, all which correlated with intelligence. This seems to support a genetic background for these differences, as regular syblings share only c. 50% of the genes that are variable in humans (can be more or less but around that median figure anyhow), while identical twins share virtually 100% of their genes.
I find this interesting because the main classical anthropometrical correlation with intelligence used to be brain size (though independent for each gender) but many of us suspected that size was not the only thing that mattered. This in fact comes to prove that the quality of wiring, something that cannot be measured in cubic centimeters of cranial capacity, also influences intelligence strongly. It will be surely long before we can discover other such "design" factors (mieline quality being again a relatively easy to measure element, yet only now studied), things like the design of the neuronal network itself, probably a less genetic and more plastic one, organized largely in life by experience, but I'm sure such factors are acting right now. Of course the quality of the materials is influential, maybe fundamental, but how they come to be organized, something that only happens within life, which is surely as dependent on education, general experience and access to knowledge, also matters. The best computer of Earth, if only programmed to add 2+2 won't be able to do anymore, while the worst one can surely do much more than that with due attention. .