About 84% of the genes are expressed in the brain. Given that humans have 20,000 genes, that means about 16,800 genes are expressed in the brain.
We should not be surprised, then, if we were to find more than a bit of variation among human brains.
We should expect to see variation in degrees of creativity vs. copying, on liberalism vs. conservatism, on selfish behavior vs. altruism, introversion vs. extroversion, leadership vs. following, variations in thinking styles, degrees of mental energy, I.Q. and flexibility of I.Q., and of course any of a variety of learning and mental disabilities. These last are of course often disabilities based on a certain accepted mean of learning and/or behavior.
I have noted in some previous posts, linked above, that each of these consists of a spectrum of behaviors, which can be placed in a 20-60-20 grouping of the two extremes and a varying middle. I suspect that the same is true of the autism spectrum as well. The numbers don't seem at first to support this, but I suspect that the number of people with Asperger's is grossly underestimated and that ADD/ADHD is properly on the spectrum, such that the true spectrum looks like this:
Indeed, recent research has found a genetic link among major depression, bipolar disorder, autism, schizophrenia, and ADHD. About 11% of the population has been diagnosed with ADHD, and while only about 0.2% of the population has been diagnosed with Asperger's (the distinction of which has been lost by being folded into autism), I strongly suspect it's more. Many we would just call "introverted" are probably on the spectrum and specifically have Asperger's. Many upon my telling them I have Asperger's insisted that, no, I was just very introverted. But as anyone on the spectrum will tell you, much of our "introversion" comes from a combination of complete mental exhaustion from having to negotiate a social environment that doesn't make much sense to us, and our not understanding how to be social, rather than a desire not to be social.
In addition to the above research, there are a number of other studies that find genetic and structural similarities between autism and schizophrenia. Indeed, autism was once considered to be a form of childhood schizophrenia. It may be that the doctors who thought that were on to something. While there do seem to be significant-enough differences between schizophrenia and autism to make the distinction worthwhile, they may be close-enough related to consider them together--at least for the purposes of this essay.
If we take these things into consideration, we have an expanded autism spectrum that includes something like 20% of the population. If that is the case, what we have here is not really a disorder, but a natural variation that contributes to social complexity and dynamics. At the other end, constituting another 20% of the population, would then be what we could consider solipsistic thinkers, who are in many ways truly opposite of autistic, as I discuss here.
Also, one may note that there are a lot of overlaps in categories. Many introverts are on the autism spectrum, and vice versa (many with ADHD may be considered extroverts because of their hyperactivity, so the correlation, in my expanded definition of autism, won't be perfect with introversion); many on the spectrum are creative and non-conformists. (It is notable that people on the spectrum, while being non-conformists, also dislike a great deal of change, while the more conformist neurotypicals are more capable of change; this tension also likely contributes to social dynamics in interesting ways that should be investigated.) Variations in thinking styles also maps well onto the solipsistic to autism spectrum.
Variations in brain structure, then, is going to be quite common. Given the number of genes involved in the brain, what should be most surprising is that so much is common among humans. This is in no small part because various streams tend to converge into the same general pathways (as described by constructal theory). This is why there can be a variety of causes of autism, with there being similarities among those who have autism (even with variations in degrees of expression). For there to be complex human societies, it would be necessary to have a variety of ways of thinking or even a variety of kinds of minds so that our societies are neither too stagnant nor too changeable. The most stable societies will be those that both honor tradition and are open to change, that change on the margins rather than abruptly.
Even though we have had literally millennia of species experience with the presence of such variation, we still nevertheless see a great deal of prejudice and discrimination against those who have variations in their thinking. This seems especially true in the postmodern period, where we have developed institutions whose job it is to separate out anyone who has a difference in the way they think, process information, etc. This institutional discrimination is very widespread today, to such a degree that you almost cannot get a job unless you are solidly in the 80% solipsistic-neurotypical range. Businesses quite often, if not almost always, actively discriminate against anyone on the autism spectrum, which is why so many on the spectrum are unemployed.
This discrimination against people who think differently comes from more recent egalitarian attitudes which insist that everyone is/must be identical. Given that these variations in mind/thinking cut across race, ethnicity, sex, gender, and sexual orientation, one can actively discriminate against mental variation even while insisting on acceptance of other categories. Worse, because these mental differences are real and are a consequence of structural differences, insistence that all children are the same and learn the same results in the development of the idea of learning disabilities and of behavioral problems.
The politically correct change of this to "learning differences" has not resulted in any real change in attitude toward those differences as being bad. And differences in processing and interacting with the world are treated as behavioral problems to be solved. But the fact of the matter is that people on the spectrum cannot and should not be expected to behave like neurotypical people, because the are literally structured differently. This isn't a matter of something superficial like culture, which can be written on any individual born into that culture, regardless of race, etc.; no, this is something deep and fundamental that cannot be so readily changed.
And even if the changes can be made--typically, forced--they always feel artificial to the person. It's much like insisting that gays can just ignore their preferences and act heterosexual; it can be done, but it will never feel quite right, and it will likely make the person feel anxious and depressed. Perhaps not coincidentally, anxiety and depression are typically part of autism.
Our societies have been formed by the majority of those not on the autism spectrum. There are obvious reasons for that--not the least of which being that those people make up 80% of the population. As a result, it is not entirely unreasonable to insist that we on the spectrum conform to them and not vice versa. Of course, this seems easy enough to a group of people for whom conformity is natural. But what they need to understand, what everyone needs to understand, is that it's not easy for us.
More, by preventing us from being ourselves--at least on occasion--I suspect that our societies are losing out on a great deal that we could and would otherwise contribute to society. Free to be ourselves, with less anxiety and depression, we may feel more up to innovating and creating and thus contributing to society in the many ways we have in the past. That's all we ask: to be allowed to be ourselves, to be allowed to contribute, to be allowed our humanity.