📧 Reading John Medina’s Substack newsletter. He’s a developmental molecular biologist, and author of Brain Rules. Each newsletter I’ve seen so far usually provides a quick summary of something unusual, occasionally jaw-dropping, about the human brain, how it works, what it does.

Issue 25 was one that stood out to me. It told the story of “Timmy”, one of the “personalities” that a patient with Dissociative Identity Disorder (aka DID, previously referred to as Multiple Personality Disorder) had.

Over time his body would appear to be inhabited by one of around 12 different personalities, if that’s the right word. Some of his personalities were allergic to orange juice. If he drank whilst one of these was active then his skin swelled up, became red and developed hives. This is not so unusual, people often have allergic reactions.

The twist is that some of his other personalities did not have this allergy. If they were in charge of his inner show at the time of drinking a glass of juice then his body wouldn’t show any of those symptoms. No sign of any allergy was to be found.

If he switched from a personality that was allergic to the juice to one that wasn’t in the middle of undergoing an allergic reaction then his hives, redness and itching would simply disappear.

It thus became apparent that what’s usually assumed to be a physical, deterministic, response to a chemical is in fact context dependent. Whilst no-one thinks we usually consciously “create” our allergic reactions, this suggests that something somewhere in our brain must unconsciously be governing them. Hence there’s a sense in which they are under our control, existing in relation to us rather than simply being an inanimate “fact of the universe” - even if we don’t know to how to consciously leverage them

“All in the mind” is a phrase that often used derogatorily, unsympathetically and dismissively to patients suffering various conditions, as if suffering situated primarily in the workings of the mind was somehow of lesser concern than suffering entirely on overtly physical phenomena. But stories like this constantly keep me wondering to what extent almost anything of note is at least somewhat “in the mind”. The book “The Expectation Effect” added a ton of counterintuitive examples towards that end.