# Proof Narrative: Adult neurogenesis occurs in the human neocortex.

## Verdict

**Verdict: DISPROVED**

The evidence is clear and settled: the adult human brain does not grow new neurons in the neocortex. Two independent studies on human tissue reached the same conclusion using the most precise method available, and no credible evidence has emerged to challenge them.

## What was claimed?

The claim is that the mature human brain -- specifically the outer layers of the cerebral cortex responsible for thought, perception, and language -- continuously produces new neurons throughout adulthood. This idea matters because it would have significant implications for brain plasticity, recovery from injury, and our understanding of cognitive aging. If true, it might suggest the brain can replenish cells lost to disease or time.

## What did we find?

The most direct test of this claim used an ingenious technique borrowed from nuclear history. Atmospheric nuclear weapons testing in the mid-20th century released a pulse of radioactive carbon-14 that was absorbed into DNA worldwide at the moment cells divided. By measuring the carbon-14 content of neurons in human tissue, scientists could determine precisely when those cells were born. When researchers applied this method to the neocortex in 2006, they found that every neuron tested had been born around the time of the individual's birth or shortly after. None showed carbon-14 signatures consistent with formation in adulthood. In the same study, patients who had received a chemical marker for cell division as part of cancer treatment provided another window: over five hundred cells in the neocortex had absorbed the marker, but not a single one turned out to be a neuron.

Seven years later, a separate study independently applied the same carbon-14 dating technique to human cortical neurons -- this time as a control measurement while studying a different brain region. The result was the same: cortical neurons are not replaced after birth at any detectable level.

Earlier reports in the late 1990s had suggested that macaque monkeys might grow new neurons in their cortex. Those findings generated real excitement, but they relied on a labeling technique that can inadvertently mark cells undergoing DNA repair -- not just dividing cells. Follow-up studies using more rigorous methods could not replicate the original results. The field now regards those early positive reports as methodological artifacts.

A deliberate search for any post-2013 study that might overturn these findings came up empty. Review articles published through 2023 continue to state that cortical neurons are not locally generated in adulthood. No research group has reported contrary evidence using carbon-14 dating or any comparable method.

## What should you keep in mind?

This verdict applies specifically to the neocortex -- the part of the brain most people have in mind when thinking about higher cognition. It does not settle the question for other brain regions. There is an ongoing, active scientific debate about whether the adult human hippocampus generates new neurons, particularly in a region called the dentate gyrus. That debate remains genuinely unresolved, with high-profile studies on both sides. The neocortical question, however, is not part of that controversy -- researchers on both sides of the hippocampal debate treat the neocortex as separately settled.

It is also worth noting that the proof addresses neurogenesis at detectable levels using current methodology. It does not exclude the theoretical possibility of neuron generation below the detection threshold of carbon-14 dating. And "no new neurons" does not mean the adult brain is static -- existing neurons form new connections constantly, and non-neuronal cells do proliferate in the adult cortex.

## How was this verified?

This claim was evaluated by identifying published peer-reviewed studies that directly tested for adult neocortical neurogenesis in human tissue using carbon-14 bomb-pulse dating, verifying that each study explicitly rejected the claim, and checking whether any subsequent research had overturned the consensus. All citations were verified by live-fetching the source URLs and confirming the quoted text. Full methodological details are in [the structured proof report](proof.md) and [the full verification audit](proof_audit.md). You can also [re-run the proof yourself](proof.py).
