Some claim the disease hydrocephalus reduces brain size by 95% but often with normal or even above-average intelligence, and thus brains aren’t really necessary. Neither is true.
Hydrocephalus is a damaging brain disorder where fluids compress the brain, sometimes drastically decreasing its volume. While often extremely harmful or life-threatening when untreated, some people with severe compression nevertheless are relatively normal, and in one case (Lorber) they have been claimed to have IQs as high as 126 with a brain volume 5% of normal brains. A few of these case studies have been used to argue the extraordinary claim that brain volume has little or nothing to do with intelligence; authors have argued that hydrocephalus suggests enormous untapped cognitive potential which are tapped into rarely for repairs and can boost intelligence on net, or that intelligence/consciousness are non-material or tapping into ESP.
I point out why this claim is almost certainly untrue because it predicts countless phenomena we never observe, and investigate the claimed examples in more detail: the cases turn out to be suspiciously unverifiable (Lorber), likely fraudulent (Oliveira), or actually low intelligence (Feuillet). It is unclear if high-functioning cases of hydrocephalus even have less brain mass, as opposed to lower proxy measures like brain volume.
I then summarize anthropologist John Hawks’s criticisms of the original hydrocephalus author: his brain imaging data could not have been as precise as claimed, he studied a selective sample, the story of the legendary IQ 126 hydrocephalus patient raises questions as to how normal or intelligent he really was, and hydrocephalus in general appears to be no more anomalous or hard-to-explain than many other kinds of brain injuries, and in a comparison, hemispherectomies, removing or severing a hemisphere, has produced no anomalous reports of above-average intelligence (just deficits), though they ought to be just the same in terms of repairs or ESP.
That hydrocephalus cases can reach roughly normal levels of functioning, various deficits aside, can be explained by brain size being one of several relevant variables, brain plasticity enabling cognitive flexibility & recovery from gradually-developing conditions, and overparameterization giving robustness to damage and poor environments, and learning ability. The field of deep learning has observed similar phenomenon in training of artificial neural networks. This is consistent with Lorber’s original contention that the brain was more robust, and hydrocephalus was more treatable, than commonly accepted, but does not support any of the more exotic interpretations since put on his findings.
In short, there is little anomalous to explain, and standard brain-centric accounts appear to account for existing verified observations without much problem or resort to extraordinary claims.