In the post to which this is a sequel, I reported remarkable research which suggests that, to the extent that our species could be seen as having domesticated itself (in a biological sense), people arguably show reduced domestication. I also pointed out that this finding beautifully fits the diametric model of mental illness, according to which autism features deficits in mentalism understood as an ability to relate to other people psychologically and socially. However, what I did not mention was the obvious implication that, if the diametric model is to be believed, sufferers from psychotic illnesses which feature hyper-mentalism such as schizophrenia (SZ) should show signs and symptoms of corresponding hyper-domestication diametrically opposite to those of the hypo-domestication associated with autism and described in the previous post.

Source: Antonio Benítez-Burraco

Having previously shown that SZ and autistic patients process language differently and exhibit distinctive oscillatory profiles when doing so, Antonio Benítez-Burraco, writing with Wanda Lattanzi, reports that “people with SZ exhibit more marked domesticated traits at the morphological, physiological, and behavioural levels,” just as the symmetry of the diametric model predicts and the figure above illustrates:

Minor dysmorphisms are typically featured in the craniofacial area of SZ patients. Indeed, facial asymmetries, particularly those arising along the midfacial junctions ... are reproducibly found in these patients … Additionally, ear shape abnormalities ... are usually observed in SZ phenotypes … Some of these features ... are considered as pathognomonic for SZ in the differential diagnosis of psychotic conditions … Anomalies in the mouth (e. . decreased tooth size, abnormal palate shape and size) are also commonly observed in … Likewise, the odds of having a psychotic disorder seem to be increased in people with shorter and wider palates … Some studies suggested a significant association between minor physical anomalies and the early onset of the disease … More generally, the odds of having a psychotic disorder seem to be increased in people with smaller lower-facial heights ... In addition, patients suffering from psychotic disorders tend to feature a more brachycephalic (i.e. shorter) skull … Brachycephaly is a frequent skull shape found in domesticated dog and cat breeds.

With regard to the brain, which is typically smaller in domestication: meta-analyses of neuro-imaging studies of the schizophrenic brain indicate a significant reduction in total brain volume, which mostly affects the hippocampus, the thalamus and the cortex, and is most pronounced in the frontal and temporal lobes. Gray matter reduction in SZ is also associated with longer duration of illness and reduced sensitivity to anti-psychotic medications.

The diametric symmetry of versus was the subject of an earlier post, and you could see castration as an epitome of the former 's fate in domestication (at least in male animals). Men with SZ have lower levels of testosterone than healthy individuals; and although not found in more aggressive patients, an inverse correlation between testosterone and negative symptoms of the disease has been described. Where oxytocin is concerned, Benítez-Burraco and Lattanzi note that a positive correlation between SZ progression and oxytocin levels in the central has been observed. They add that treatment with oxytocin improves verbal learning tasks in SZ patients, but also exacerbates negative symptoms of the disease.

As I pointed out in the previous post, domestication profoundly affects reproductive cycles in animals, and delayed is associated with greater severity of negative schizophrenic symptoms in men. In women higher negative symptom scores and greater functional impairment correlate with later age of menarche, and nearly 50% of women with SZ have irregular periods.

—along with its opposite, given selection for docility in domestication—is a key behavioural factor in "domestication syndrome." Aggressive behaviour is frequent in SZ, and may be associated with . Interestingly though, there appears to be no positive correlation between aggression and neuro-psychological performance in patients.

As I explained in the previous post, the effectiveness of the hypothalamus-pituitary axis (HPA) is reduced in domestication, and in SZ patients a decrease in HPA sensitivity correlates with the severity of negative symptoms. According to some authorities, SZ is characterised by an attenuated HPA response to social : despite a normal stress hormone production rate, schizophrenics have lower cortisol levels than controls, both in anticipation of and after exposure to social stress.

With regard to neoteny, or the retention of immature features that is symptomatic of domestication, it is noteworthy that, in striking contrast to autistic people, schizophrenic patients exhibit lower weight and reduced head circumference at birth, along with slow growth and small size in .

Lastly, concerning pigmentation, which is often reduced in domestic animals, an association between SZ and albinism has been occasionally reported. Low vitamin D levels in SZ patients correlate with the severity of psychotic symptoms, and hyper-pigmentation is a typical side-effect of neuroleptic drugs used in treatment of SZ.

As I explained in the original post on autism, neural crest (NC) cells seem to be the origin of most of the signs and symptoms of domestication in vertebrates, and Benítez-Burraco and Lattanzi note that NC cell pathology is implicated in Di George and Velocardiofacial syndromes, both of which are associated with SZ—indeed the latter carries the second highest genetic risk factor for currently known.

Nearly 30% (20 out of 67) in an extended list of candidate for domestication play some role in the development of SZ, suggesting in turn that the 5% of human genes that have been associated with SZ are over-represented among genes related to domestication. Many of the genes in question encode transcription factors and modulators, which in their turn modulate the expression of genes with far-reaching and varied roles in development and in language processing (below).

Source: Benítez-Burraco and Lattanzi in press.

These authors conclude that “Overall, these disparate lines of evidence suggest that the schizophrenic phenotype might result in part from a dysfunction of the NC and provides additional support to the view of SZ as a ‘hyperdomesticated’ condition.”

(With thanks to Antonio Benítez-Burraco for his help and particularly for providing the illustration at the top specially for this post. Also see his post, which provides further details.)