Why Human Brains Vary

Many people assume human brains vary genetically and genetic variation maps to races. But the races are not real and genetic variation can't explain brain differences. Because, dear reader, brains don't work that way.

Let's look just at the brain part of this problem.

A Repost

There are between 50 and 100 billion neurons in the human brain, and every one is connected to a minimum of one other neuron to produce about 100 trillion connections. So when we are thinking about how the brain is wired up, we have to explain how so many connections can be specified to make the brain work.

There are thousands of genes that seem to be expressed mainly or exclusively in the brain ... perhaps as many as 10,000 (or about half the genes that are active in the human genome) ... but this vast difference between number of connection and number of genes is true to nearly the same extent for all mammal brains. A human brain has way more connections (and much more "higher cognitive function") than a mouse brain, but with about the same number of genes, There may be some unique added genes in the human, but the number of additional brain circuits required to add human language and cognitive function to a mouse can not be explained by more genes unless individual genes do not do much in the way of detail.

All human populations over long(ish) evolutionary time are subjected to similar selective pressures to have a smaller brain. Large brains in humans kill mothers and children in birth. Death in childbirth is, in fact, higher for humans in a "natural state" than other mammals. The large brain is being selected against to a significant degree, or at least, it is safe to assume this.

However, large brains persist. There is some literature suggesting that some "races" have smaller brains than others. As far as I know these assertions are very suspicious, and while brain size varies across different samples, there is no reliable data suggesting that there are major population level differences in human brain size. Some of the differences that have been asserted in the literature have involve very poor data and very inappropriate manipulation of the data to make it look like there are significant population differences in brain size.

So, humans have whopping big brains, we should not have such large brains from the perspective of natural selection unless they are conferring some advantage to offset childbirth related mortality, and the number of neurons and connections, and overall complexity of human brains may be affected by genetics, but it is not possible for these connections to be specified in any level of detail by genes. Indeed, only rough patterns could be stipulated by genetic programming, and in comparing the anatomy of normal human brains, we do not see differences between populations.

As an aside: There certainly are genetic abnormalities that cause abnormalities in human brains, but just as a gene that cause a person to be born without legs is not assumed to have alleles that affect running abilities, a gene that if "broken" causes a "broken" brain can not be assumed to be a gene with allelic variation that affects day to day normal brain function. Genes don't work that way, bodies don't work that way.

So we are left with the question: How does your brain get to the point where it functions? You may not realize this, but there is an ancillary question as well: How come highly smart human like brains seem to not evolve very often on this planet?

The way mammalian brains form is generally the same: Genes specify, using signaling chemicals, overall positioning of proliferating neurons, which then over reproduce and over-connect. So, all individuals start out with brains with many extra neurons and many extra connections. This infantile mammalian brain is so over-produced in terms of cells and connections that it can't function well. It is also short on insulating fats that normally cover the axons (the parts that connect neurons to other neurons).

Then over time connections break and neurons die. This process mainly depends on input. So, neural connections that are not used die. Neural connections can thus, during development, be formed in response to the environment, where the environment includes other parts of the brain, the body the brain is in, and the surrounding physical environment the body lives in.

And, in the case of humans and presumably to varying degrees some other mammals, the brain is shaped (sculpted, really) by the culture in which it grows.

This process of shaping the brain based on the culture within which exists is a Darwinian process a the neural (not genetic) level because neurons have different chances of survival depending on this environment. The degree to which this is an externally caused cultural process is well exemplified by the way reading and writing capacities form in the brain. We have brain regions specific to these functions, which could not have evolved and can not be specified by genes, which differ between individuals in how well they function (how "reading able" someone is) based on their experience, and the kind of language being read or written determines which region of the brain is shaped for this function. Some languages use mainly temporal regions and other languages use mainly occipital regions. A person who can read and write in both kinds of languages can lose, due to brain damage such as a stroke, the capacity to read or write one of the languages with the other left intact because of this physical separation.

Culture is not only required to create a functioning human brain, but culture can create all different kinds of brains. There are probably limits as to how different brains can be based on cultural differences, but finely tuned tests can be constructed to measure some of these differences. In my view, Western Middle Class Intelligence Tests are one such measure. It is said that such tests have been redesigned to remove all biases, but the same people who make that claim have also made other claims about human brains that show that they have little concept of how brains develop or how they may differ from environmental causes.

Another potential cause of difference in brains that is also environmental is dietary. It is probably true that almost everyone's brain is challenged by shortages of energy, oxygen, and key nutrients at various stages of growth and development because the brain is so demanding. But individuals who have had more such challenges may well end up with a brain lacking adequate myelination in some areas, or damaged glial functioning or some other problem that can impair brain function.

Home environment, linguistic environment, diet, and other environmental factors probably sum to having a much greater effect on brain development (and thus on various tests of brain function) than genetic factors. Or at least, in the absence of overwhelming evidence (or even modest evidence, for that matter) to the contrary, environmental causes of variation in the brain, which is an environmentally shaped organ, can safely be assumed to be paramount based on everything we know about brain development and function.

The best book I know of to explore this issue is not too current but is still quite good. Supporting evidence for everything given in this post will be found in this source: The Symbolic Species: The Co-Evolution of Language and the Brain by Terry Deacon.