The brain develops bottom-up, from survival systems to reasoning

Perry organizes the brain, for clinical purposes, into a rough hierarchy: the brainstem, which regulates basic survival functions like heart rate and body temperature and develops first; the limbic system, which handles emotion and attachment; and the cortex, which handles complex reasoning and develops last and slowest, continuing to mature well into a person's twenties.

Because lower brain regions develop earlier, trauma and neglect during infancy and early childhood have outsized effects on those foundational systems, which then constrain how well the higher, later-developing regions can function — a shaky foundation limits what can be safely built on top of it, regardless of how much later nurturing arrives.

This hierarchy explains why some traumatized children can discuss their experiences articulately and rationally (cortex intact) yet still react to triggers with disproportionate physiological panic (dysregulated brainstem and limbic responses) — the reasoning brain and the survival brain aren't always in sync, and trauma often hits the latter hardest.

Takeaway: you can't reason a child out of a fear response that was built into a part of the brain reasoning doesn't reach.