Over human evolutionary history, our brain size has increased dramatically as our cognitive capabilities have grown. Even among modern humans, brain size accounts for around 10 per cent of the difference in intelligence scores between individuals. Big brains may simply have more neurons and so greater processing power, or the neurons may possess more bulky insulating white matter, allowing them to communicate faster.
Nevertheless, when it comes to brains, size isn’t everything. “Neanderthals had bigger brains than we do, and they are extinct,” says Rex Jung at the University of New Mexico. “Men also have bigger brains than women. Does that mean men are smarter than women? Certainly not.”
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The truth about intelligence: A guide for the confused
Our thinking on human intellect is clouded with misinformation. But the latest science of intelligence is surprisingly enlightening
In fact, brain structure is a more reliable marker of smarts than brain size. On average, women have thicker cortices – the wrinkly, outer layer of the brain, responsible for higher-level functions – and thicker cortices have been associated with higher IQ scores. “All the wrinkles and convolutions allow more of that computational capacity to fit in,” says Jung. Men, meanwhile, tend to have bigger sub-cortical regions, including the hippocampus, which is involved in memory and spatial awareness, and the amygdala, which handles emotions and decision-making, although that . There is also more variability in these measurements between men than between women – interesting because, although there is no overall difference in intelligence between the sexes, men tend to be .
Then there’s connectivity. According to perhaps the best-supported theoretical model of intelligence, our cognitive abilities are highly reliant on a brain network linking the frontal lobes – associated with planning, organising and reasoning – with the parietal lobes, which collect and organise perceptual information. These areas are most active when people perform IQ tests – although, interestingly, less so in people with higher IQs, which could indicate that their brains are working more efficiently. Strong white matter connections between these two lobes also correlate with better performance – particularly in subtests requiring mental speed and reasoning.
Yet other studies have fingered the chemical composition of the brain. For instance, higher intelligence is linked with high levels of N-acetyl-aspartate – a substance associated with neural health and metabolism – in a brain area that coordinates sensory and visual information.
Is humanity becoming more stupid?
Taken as a whole, it looks like humans are getting cleverer – a phenomenon known as the Flynn effect. A 2015 study, which examined test results from 48 countries, found that IQ scores have . The biggest gains were in developing countries, including India and China. This suggests the Flynn effect is linked to improvements in social conditions including nutrition and education.
However, certain aspects of intelligence may be declining. When the researchers looked more closely they discovered that while short-term memory scores have risen, working memory is going in the other direction. Working memory involves manipulating information, and is among several cognitive abilities that decline as we age, possibly because . The researchers also noticed an increase in the proportion of people aged over 60 sitting intelligence tests.
This suggests that, as human populations age, our average IQ will decrease. Then again, the gap between IQ scores in developed and developing countries still stands at around three points, and is likely to close up as social conditions improve further. Is humanity becoming more stupid? Not yet – but it’s probably only a matter of time.
This article appeared in print under the headlines “Is humanity becoming more stupid” and “Are bigheads smarter than pea brains?”
