One interesting thing I took from Evolutionary Psychopathology was a better understanding of the diametrical theory of the social brain.

There’s been a lot of discussion over whether schizophrenia is somehow the “opposite” of autism. Many of the genes that increase risk of autism decrease risk of schizophrenia, and vice versa. Autists have a smaller-than-normal corpus callosum; schizophrenics have a larger-than-normal one. Schizophrenics smoke so often that some researchers believe they have some kind of nicotine deficiency; autists have unusually low smoking rates. Schizophrenics are more susceptible to the rubber hand illusion and have weaker self-other boundaries in general; autists seem less susceptible and have stronger self-other boundaries. Autists can be pathologically rational but tend to be uncreative; schizophrenics can be pathologically creative but tend to be irrational. The list goes on.

I’ve previously been skeptical of this kind of thinking because there are many things that autists and schizophrenics have in common, many autistics who seem a bit schizophrenic, many schizophrenics who seem a bit autistic, and many risk factors shared by both conditions. But Del Giudice, building on work by Badcock and Crespi presents the “diametrical model”: schizophrenia and autism are the failure modes of opposing sides of a spectrum from high functioning schizotypy to high functioning autism, ie from overly mentalistic cognition to overly mechanistic cognition.

Schizotypy is a combination of traits that psychologists have discovered often go together. It’s classified as a personality disorder in the DSM. But don’t get too caught up on that term – it’s a disorder in the same sense as narcissistic or antisocial tendencies, and like those conditions, some schizotypals do very well for themselves. Classic schizotypal traits include tendency toward superstition, disorganized communication, and nonconformity (if it sounds kind of like “schizophrenia lite”, that’s not really a coincidence).

Typically schizotypals are supposed to be paranoid and reclusive, the same as schizophrenics. But the diametrical model tries to downplay this in favor of noting that some schizotypals are unusually charismatic and socially successful. I am not exactly sure where they’re getting this from, but I cannot deny knowing several extremely charismatic people with a lot of schizotypal traits. Sometimes these people end up as “cult leaders” – not necessarily literally, but occupying that same niche of strange people who others are drawn toward for their unusually confident and otherworldly nature. Some of the people I know in this category have schizophrenic first-degree relatives, meaning they’re probably pretty loaded with schizotypal genes.

Schizotypals, according to the theory, have overly mentalistic cognition. Their brains are hard-wired for thinking in ways that help them understand minds and social interactions. When this succeeds, it looks like an almost magical understanding into what other people are secretly thinking, what their agendas are, and how to manipulate them. When it fails, it fails as animism and anthropomorphism: “I wonder what the universe is trying to tell me by making it rain today”. Or it fails as paranoia through oversensitivity to social cues: “I just saw him twitch his eye muscle slightly, which can sometimes mean he’s not interested in what I’m saying, and in the local status game that could mean that he doesn’t think I’m important enough, and that implies he might think he’s better than me and I’m expendable…”

Autism, then, would be the opposite of this. It’s overly mechanistic cognition, thinking in terms of straightforward logic and the rules of the physical world. Autistic people don’t make the mistake of thinking the universe is secretly trying to tell them something. On the other hand, after several times trying to invite a slightly autistic woman I had a crush on to things, telling her how much I liked her, petting her hair, etc, she still hadn’t figured out I was trying to date her until I said explicitly “I AM TRYING TO DATE YOU”. So not believing that you are secretly being told things has both upsides and downsides.

Autistic people are sometimes accused of looking for a set of rules that will help them understand people, or the secret cheat code that will make people give them what they want. I imagine an autistic person asking something like “What is the alternative?” This is the kind of thought process that usually works on stuff: figure out the rules that govern something, find a way to exploit them, and boom, you’ve landed a rocket on the moon. How are they supposed to know that human interaction is a bizarre set of layered partial-information games that you’re supposed to solve by looking at someone’s eye muscle twitches and concluding they’re going to steamroll over you to get a promotion at work?

Is this true? There’s…not great evidence for it. I’ve never seen any studies. There’s certainly a stereotype that brilliant engineers are not necessarily the most socially graceful people. But I know a lot of people who combine excellent technical skills with excellent social skills, and other people who are failures in both areas. So probably the best that can be said about this theory is that it would be a really neat way to explain the patterns of similarities and differences between schizophrenia and autism.

In this theory, both high-functioning autism (being good at mechanistic cognition) and high-functioning schizotypy (being good at mentalistic cognition) may be good things to have. But the higher your mutational load is – the less healthy your brain, and the fewer resources it has to bring to the problem – the less well it is able to control these powerful abilities. A schizotypal brain that cannot keep its mentalistic cognition yoked to reality dissolves into schizophrenia, completely losing the boundary between Self and Other into a giant animistic universe of universal significance and undifferentiated Mind. An autistic brain that cannot handle the weight of its mechanistic cognition becomes unable to do even the most basic mental tasks like identify and cope with its own emotions. And because in practice we’re talking about shifts in the complicated computational parameters that determine our thoughts and personalities, rather than the thoughts and personalities directly, both of these conditions have a host of related sensory and cognitive symptoms that aren’t quite directly related.

So here the reason why autism and schizophrenia seem both opposite and similar to each other is because they’re opposite (in the sense of being at two ends of a spectrum), and similar (in the sense that the same failure mode of high mutational load and low “mental resources” will cause both).

If you’re thinking “it sounds like someone should do a principal components analysis on this”, then Science has your back (paper, popular article). They find that:

Consistent with previous research, autistic features were positively associated with several schizotypal features, with the most overlap occurring between interpersonal schizotypy and autistic social and communication phenotypes. The first component of a principal components analysis (PCA) of subscale scores reflected these positive correlations, and suggested the presence of an axis (PC1) representing general social interest and aptitude. By contrast, the second principal component (PC2) exhibited a pattern of positive and negative loadings indicative of an axis from autism to positive schizotypy, such that positive schizotypal features loaded in the opposite direction to core autistic features.

In keeping with this theory, studies find that first-degree relatives of autists have higher mechanistic cognition, and first-degree relatives of schizophrenics have higher mentalistic cognition and schizotypy. Autists’ relatives tend to have higher spatial compared to verbal intelligence, versus schizophrenics’ relatives who tend to have higher verbal compared to spatial intelligence. High-functioning schizotypals and high-functioning autists have normal (or high) IQs, no unusual number of fetal or early childhood traumas, and the usual amount of bodily symmetry; low-functioning autists and schizophrenics have low IQs, increased history of fetal and early childhood trauams, and increased bodily asymmetry indicative of mutational load.

If men have much more autism than women, shouldn’t women have much more schizophrenia than men. You’d think so, but actually men have more. But men have greater variability in general, which means they’re probably more likely to satisfy the high mutational load criterion. So maybe we should instead predict that women should have higher levels of high-functioning schizotypy. Studies show women do have more “positive schizotypy”, the sort being discussed here, but lower “negative schizotypy”, a sort linked to the negative symptoms of schizophrenia.)

Something that bothered me while I was writing this: famous mathematician John Nash was schizophrenic. Isn’t that kind of weird if schizophrenia is about an imbalance in favor of verbal/personal and against logical/mathematical thinking?

There are exceptions to everything, and we probably shouldn’t make too much of one case. But I find it striking that Nash’s work was in game theory: essentially a formalization of social thinking, and centered around the sort of paranoid social thinking of figuring out what to do about how other people might be out to get you. This is probably just a coincidence, but it’s pretty funny.