In a psychologist's quiet office, a fourteen-year-old fills out a questionnaire about childhood. The scores climb, and on the MRI scans the amygdala climbs with them: a small node deep in the brain that teaches us to fear and to remember danger. In a rat held in a narrow tube for three hours a day, the same structure grows faster than in its calm counterparts. But here is the oddity: in older people who endured abuse or neglect as children, the amygdala is smaller, not larger. How do we reconcile these facts? The answer hides in age.
Picture a spring in a mechanical watch. In youth it gets wound too tight: early stress makes the amygdala grow at an accelerated pace, as if the brain were rushing to prepare for a dangerous world. In rats after chronic restraint, the volume of this region increases especially noticeably; in adolescents with high childhood trauma scores, growth is steeper from 14 to 23. But a spring that has been overwound loses its elasticity over time. By ages 40-69, people with a harsh childhood have a smaller amygdala than those who grew up calm. This is not a contradiction but a single trajectory: first acceleration, then exhaustion.
To see this trajectory in full, the scientists compared three groups, measuring amygdala volume with the same MRI method. Wistar rats were scanned before stress, immediately after, and a month later. Adolescents from the European IMAGEN cohort were followed from 14 to 23, with scans matched to their reports of childhood trauma. Adults from UK Biobank, about 25,000 people, were examined at ages 40-69. The results fell into a single picture: in adolescents with trauma, the amygdala grows faster; in adults with trauma, it is smaller. Age turned out to be the key that opens the lock of contradictions.
This work explains why previous studies produced contradictory results: they caught people at different points along the same trajectory. The "amygdala burnout" model suggests that early stress makes this structure grow faster, but then its volume drops below the level of people without stress. For science, this means that when studying the consequences of stress, one must account for age at the time of measurement, not simply the presence or absence of adverse experience. A spring overwound in childhood compresses more tightly by adulthood than in those who grew up calm.
What is still missing is longitudinal data on people over 40: we do not know exactly how amygdala volume changes after the peak. Three possible scenarios: a sharp drop, a convergence toward the norm, or a parallel decline. To choose among them, repeated MRI scans of the same people over decades are needed. It is also important to find out what role stress in adult life plays: it may amplify or, conversely, smooth out the effects of childhood experience. But one thing is already clear: early help for children from disadvantaged families is not only about well-being, but also about brain health forty years later.
In normal development, the human amygdala increases in volume by 40% between ages 8 and 18, and a third of that growth comes from the maturation of neurons in the basolateral nucleus. In rats after chronic stress, the volume of this region grows even faster, visible within just a few weeks.
Daniel Keyes's novel Flowers for Algernon shows how accelerated intellectual development turns into its rapid decline. The "amygdala burnout" model draws a similar trajectory for the emotional brain: early stress speeds up growth but drains the resource.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.