# Proof Narrative: Over 80% of the brain's neurons are located in the cerebellum.

## Verdict

**Verdict: PROVED**

It sounds counterintuitive, but the tiny cerebellum — that dense, cauliflower-like structure tucked at the back of the skull — holds the vast majority of the brain's neurons, and the numbers back it up.

## What was claimed?

The claim is that more than 80% of all the neurons in the human brain are packed into the cerebellum. Most people think of the cerebrum — the large, wrinkled outer brain — as the "main" brain, so the idea that a structure accounting for just 10% of the brain's weight could house over four-fifths of its cells strikes many as surprising, or even implausible. Understanding where neurons actually live matters for anyone thinking about how the brain processes movement, learning, or disease.

## What did we find?

The numbers come from a landmark 2009 study published in *Frontiers in Human Neuroscience* by neuroscientist Suzana Herculano-Houzel, who used a method called isotropic fractionation to count neurons directly rather than estimating from volume. Her data: the human brain contains roughly 86 billion neurons in total, and 69 billion of those are in the cerebellum. That works out to 80.23% — just barely over the 80% mark.

Two independent peer-reviewed papers corroborate the figure. A 2016 review of 150 years of cell-counting methodology in the *Journal of Comparative Neurology* states plainly that "the cerebellum contains about 80% of all neurons in the human brain." A separate 2010 cross-species study by Herculano-Houzel and colleagues, comparing everything from mice to macaques to humans, also places the human figure at 80%.

The reason for the discrepancy between the cerebellum's modest size and its enormous neuron count is density. Cerebellar granule cells are among the smallest and most tightly packed neurons in the nervous system. The cerebrum is bigger and more metabolically demanding per neuron, but it is far less densely packed. Volume and neuron count simply don't track each other the way most people assume.

## What should you keep in mind?

The margin here is thin. The computed value is 80.23% — just 0.23 percentage points above the 80% threshold. The original study reports measurement uncertainties of roughly ±8 billion for total neurons and ±6.6 billion for cerebellar neurons, which are much larger than that margin. The scientific literature consistently describes the figure as "about 80%," not "strictly over 80%." The claim is proved on the point estimates as reported, but treating it as a precise hard fact rather than an approximate one would be a mistake.

There is also a competing measurement. A 2013 study by Andrade-Moraes and colleagues, using a different methodology, estimated only 54 billion cerebellar neurons — which would place the cerebellum at around 63% of total neurons, well below 80%. That study hasn't displaced the Azevedo/Herculano-Houzel count as the field's reference figure, but it is a real scientific alternative, not noise.

One definitional point matters: "brain" here means the organ inside the skull — cerebrum, cerebellum, and brainstem — and excludes the spinal cord. If you include the spinal cord's roughly 1 billion neurons, the cerebellum's share drops to about 79.3%, just under 80%. The claim is true under standard neuroanatomical usage.

## How was this verified?

This claim was evaluated by fetching the cited sources live, extracting neuron counts directly from quoted text, computing the percentage independently, and cross-checking the result against two separately authored peer-reviewed papers. You can read the full breakdown in [the structured proof report](proof.md), inspect every citation and computation step in [the full verification audit](proof_audit.md), or [re-run the proof yourself](proof.py).