Two ordinary people suddenly go insane together. It’s a premise we enjoy from a safe distance—because surely it could never be us. I’m not so sure. Shared madness isn’t rare, it isn’t aberrant, and it sits a lot closer to ordinary love than we’d like to think. Strip away the spectacle and what’s left is the most common thing in the world: two lonely people who found a home in each other.

• Folie à deux: the madness of two — the article that inspired this lecture.
• Four models of psychopathology — how we decide what counts as “abnormal”, and why a benign shared delusion slips past all of it.
• The loneliness epidemic and Explaining group dynamics — why social isolation is so dangerous for us.
• It’s Not Social Media, Life Is Just Worse — a companion lecture on modern isolation.
• Successful Prophets — the same connection mechanism scaled from the pair to the group.

• Ursula and Sabina Eriksson (the Swedish sisters).
• Folie à deux; Jules Baillarger, Charles Lasègue and Jean-Pierre Falret.
• Shared psychotic disorder (the undiagnosis quote), and the intimacy-in-isolation qualification.
• The Japanese family case (shared delusional hallucination); delusional parasitosis; shared pseudocyesis.
• The Tromp family (BBC, Mamamia).
• Theranos: Elizabeth Holmes, Sunny Balwani, and Bad Blood.
• Group polarisation and risky shift.
• Gang-stalking, Morgellons, and mass psychogenic illness.