What causes mental illnesses like schizophrenia and autism? We have long known that both tend to run in families and that if one of two identical twins has such a disorder, there is a much higher than average probability that the other will too. Autism is sometimes associated with genetic syndromes, such as Rett, Down, and Turner’s, Phenylketonuria, and Tuberous Sclerosis. The clearest single-gene cause of autism spectrum disorder (ASD) is Fragile X syndrome, with a wide range of severity in symptoms and 25-47 per cent of affected males meeting the criteria for autism. But neither autism nor schizophrenia obeys classical Mendelian laws of inheritance in the way that Cystic Fibrosis or some types of colour blindness do.
According to the so-called imprinted brain theory, the paradoxes can be explained in terms of the expression of genes, and not simply their inheritance. Imprinted genes are those which are only expressed when they are inherited from one parent rather than the other. The classic example is IGF2, a growth factor gene only normally expressed when inherited from the father, but silent when inherited from the mother. According to the most widely-accepted theory, genes like IGF2 are silenced by mammalian mothers because only the mother has to pay the costs associated with gestating and giving birth to a large offspring. The father, on the other hand, gets all the benefit of larger offspring, but pays none of the costs. Therefore his copy is activated. The symbolism of a tug-of-war represents the mother’s genetic self-interest in countering the growth-enhancing demands of the father’s genes expressed in the foetus—the mother, after all, has to gestate and give birth to the baby at enormous cost to herself.
Mental disorders can be located along a dimension of mentalism (aka ‘theory-ofmind,’ ‘folk-psychology’ or ‘people skills’) defined as our evolved ability to comprehend others’ actions and behaviour in purely mental terms (such as intention, belief, desire, emotion etc.). Autistics, notoriously, are poor where mentalistic skills like inferring intention or understanding false belief are concerned. ASDs therefore belong on the hypo-mentalistic side of the continuum. However, what we would now term psychotic spectrum disorders (PSDs) can be typified as hyper-mentalistic: paranoid schizophrenics, for example, symptomatically over-interpret intention either positively in erotomania (delusions that others are in love with you) or negatively in delusions of persecution. They also entertain bizarre false beliefs about themselves and others, and generally exhibit excessive mentalism, often enshrined in quasi-religious or mystical delusions. Indeed, the symptoms and signs of autism and psychoses like paranoid schizophrenia exhibit a remarkable pattern of antithesis.
The concepts of hypo- and hyper-mentalism readily explain the last item: age of onset. Typically, this is early childhood for autism but late adolescence or adulthood for schizophrenia: a difference which up until now has lacked an obvious explanation. But the fact that you have to develop normal mentalistic skills before you can over-develop them to the point of psychosis readily explains why the mentalistic deficits of autism are apparent in childhood and why the hyper-mentalism of psychosis can only become fully apparent much later. Mentalism appears to be the key to social behaviour because autistics are notably non-social in the sense that they typically lack social skills and have impoverished social lives with few if any friends, little interest in group activities, and muted emotional responses such as empathy and interest in others.