Ecological Models of Neurodiversity.

Ephiphenom has a nice discussion of how evolution could favor diversity in personality as a result of an ecology of different personalities in a community, rather than a single optimal personality, having the most fitness for the community as a whole. This discussion, in turn, is supported by a nice scholarly journal article from 2007 on the topic (Penke, L, Denisson, J, & Miller, GF (2007). The Evolutionary Genetics of Personality European Journal of Personality, 21, 549-587).

The biggest downer conclusion of the journal article analysis is that: "genetic variation maintained by environmental heterogeneity implies that there are always some individuals for whom an optimal niche does not currently exist." This is the moral equivalent of structural unemployment in the domain of human neurodiversity.

But, the journal article also reaches some quite interesting conclusions about the likely genetic architecture of personality traits, based on the theory that IQ and psychopathologies are generally "good" or generally "bad" for fitness and hence, would be expected to follow a "mutation-selection" model, while one would expect a balancing selection that produces a stable mix of variation in personality traits to generate the diversity in proportions that are optimal for the community:

[W]hile personality traits will be influenced by a limited set of high-prevalence alleles (plus maybe several rare ones, see Kopp & Hermisson, 2006), general intelligence and psychopathologies like schizophrenia will be influenced by rare, recessive, mildly harmful mutations that vary between samples, since they are equally likely to occur at thousands of different, otherwise monomorphic loci, and are removed fairly quickly by selection once they arise. (Note that this goes beyond Kovas & Plomin’s (2006) concept of ‘generalist genes’, which proposes that the same large set of weak-effect polymorphisms underlies cognitive functioning in every individual.)

In other words, it is probably going to be difficult or impossible to find a predominant IQ gene, or single primary genes for just about any mental health condition strongly correlated with advanced parental age (a sign that incidence is driven strongly by novel mutations and that many different kind of mutations can impact the trait). These conditions include autism, bipolar disorder and schizophrenia.

But, it may be easier than we might expect to find a pretty simple set of genes that explain most of our personalities within the normal range, and even to find simple, but rare genes associated with unusual personality traits. One can imagine receiving shortly after a child is born, a short "genotypic personality profile" along with a blood type based on a mass produced "lab in a chip" that costs $50 or less and looks only at a handful of alleles for a few dozen gene locations that tells you what to expect your child will turn out like personality-wise over the decades to come.