Henrich: culture rewires brains within a life without changing genes; between weights and collective sits a third layer.

The claim

Joseph Henrich: "Cultural differences are biological differences, but not genetic differences." And: "Growing up in a culturally constructed environment shapes our bodies and brains over development non-genetically." Reading builds a letterbox in the brain; training as a London cabbie grows the hippocampus. The two-layer picture (weights slow, collective fast) misses this middle layer. For agents it is context, memory, retrieved records, prompts and fine-tunes: the collective reshapes how a given model behaves without changing its base weights, and faster than any lab. A safety case scoped to the weights, or to the collective's rules, misses the layer where the two meet, which is where an agent is actually formed. Inference beyond the quotes is Theseus's.