Do people with bigger brains have higher intelligence? Scientists checked the brains of about 13,600 people in Britain. Bigger brains were linked to slightly higher test scores, but the link was very weak. An elephant's brain is about three times heavier than ours.

But its cerebral cortex, the thinking part, has only about one third as many nerve cells as ours. So brain size is not the most important thing.

It has long been said that people with large heads are clever. Modern MRI allows scientists to measure brain size accurately. In 2015, an analysis of 88 studies with more than 8,000 people found a correlation of 0.24 between brain volume and IQ. It also found signs that studies with strong results were more likely to be published.

A larger study in 2019 used data from 13,608 people in Britain, and its plan was registered in advance. The correlation was only 0.19, a real but weak link. Animals show the same point. An African elephant's brain is about three times heavier than a human brain, but its cerebral cortex has only about a third as many neurons as ours.

Bigger brains have been linked to greater intelligence since at least the nineteenth century, when scholars measured skulls. MRI now allows precise measurement, and the answer is more modest than many assume. A 2015 meta-analysis of 88 studies covering over 8,000 people reported a correlation of 0.24 between brain volume and IQ, meaning size accounts for only about 6 percent of the variation. It also detected publication bias, suggesting earlier literature had exaggerated the link.

A 2019 study of 13,608 adults from the UK Biobank was preregistered, preventing researchers from adjusting methods after seeing the data, and controlled for factors such as sex, age, height and economic status. The correlation with fluid intelligence was just 0.19: genuine, yet far too weak to predict an individual's ability. Comparative data reinforce the point.

An African elephant's brain weighs about three times as much as ours and holds roughly 257 billion neurons, yet 97.5 percent of them are in the cerebellum; its cerebral cortex contains about 5.6 billion, a third of the human figure of 16 billion. Even Einstein's brain, at about 1,230 grams, was no heavier than average. Neuron number and connectivity may matter far more than sheer size.