The average human brain size is growing — but that doesn't exactly mean we're smarter

Humans owe our impressive intellect to our large brains, which are unusually sophisticated thanks to evolution. The first surge in our brain size occurred between 2 million and 800,000 years ago, when our species' increase in territory and physical size caused our brains to literally grow. As the climate changed between 800,000 and 200,000 years ago, human brains became even more complex so people could adapt to their new environments. The neocortex in particular expanded during this time, since this region of the brain controls higher-order brain functions including language, motor commands, sensory perception and spatial reasoning.

Yet while these past periods of growth occurred over thousands of years, a recent study in the journal JAMA Neurology reveals that the human brain has in recent years grown considerably on a mere decade-by-decade basis.

It all comes down to the town of Framingham, Mass. Since 1948, scientists have observed more than 5,200 participants across three generations to learn about their cardiovascular health. Over time the so-called Framingham Heart Study yielded a bounty of information about heart-related health matters, ranging from the importance of diet and exercise to the use of medications like aspirin. Yet the researchers behind the new article looked instead at brain-related data, drawing from a cohort of 3,226 participants born between 1925 and 1968. The MRIs of these patients revealed that, as each decade progressed, the comparative brain sizes of the human subjects became larger and larger.

"In summary, our results indicate that [intracranial volume], white matter volume, and hippocampal volume as well as cortical surface area have increased over decades of birth ranging from 1930 to 1970," the authors explain.

While this data may make it seem like humans are simply getting smarter, the news here may actually be even more hopeful. Diseases like Alzheimer's, strokes and other cognitive impairments continue to bedevil doctors seeking a cure. Because research indicates that "early life environmental influences are more likely contributors" than genetics to intracranial volume and brain size, it seems reasonable to assume that improvements in quality of life during the mid-20th century caused the growth in human brains.

These bigger brains are healthier brains — and that, in turn, may offer a clue as to how this research can be used to treat brain diseases.

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"Life course perspectives emphasize the impact of early life experiences on brain health that also translate into larger brain structures and reduced risk for later-life dementia through improved reserve," the authors write. "Similarly, efforts to improve cardiovascular health during adulthood that occurred over the time duration of this study are associated with reduced incidence of cognitive impairment and dementia, indicating that modifying these factors could also serve to improve resistance to late-life dementia."

Dr. Charles DeCarli, the lead author of the study and a distinguished professor of neurology at the University of California, Davis, explained in a press statement that a bigger brains seem to be less vulnerable to the ravages of neurodegenerative diseases. Salon reached out to DeCarli for comment and did not hear back.

“Larger brain structures like those observed in our study may reflect improved brain development and improved brain health,” DeCarli said. “A larger brain structure represents a larger brain reserve and may buffer the late-life effects of age-related brain diseases like Alzheimer’s and related dementias.”

Jeremy M. DeSilva, an associate professor of anthropology at Dartmouth College who was not involved in the study, told Salon that the new research has important social justice implications.

"This study suggest that a key organ is sensitive to poor environmental conditions, which should motivate us as a society to reduce food insecurity, especially in children," DeSilva said. DeSilva also said that a 2018 study from South Korea in the American Journal of Biological Anthropology had the same findings — brain volume increasing from generation to generation — but expressed skepticism about whether human brains overall have grown by nearly 7%. DeSilva said that "it is well-established that brains shrink as we age" and that "while the authors controlled for age, it is almost certain that the difference in brain size between the different cohorts is because of this phenomenon."

Although he acknowledged that improvements in nutrition could have resulted in brains getting slightly larger, "I’d be surprised if it was as large as the nearly 7% they report in their study."

Even if that is happening, DeSilva said that this would not mean humans are getting smarter.

"Brain volume is very weakly related to any measure of intelligence," DeSilva said. "For example, Einstein’s cranial capacity was 1,291 cc, smaller than the average cohort in this study and he was, well, Einstein. Additionally, Pleistocene humans and Neanderthals had larger brain volumes than humans today but there is no evidence that they were substantially more intelligent than we are."

Instead of suggesting human brains are growing as we become smarter, the study's main contribution to the field of brain science is in offering hope that its findings could one day treat dementia. Indeed, this is the third 2024 study that has offered scientists new ways of understanding how to treat Alzheimer's.

A pair of studies, both published in the journal Nature earlier this year, further illuminated the link between sleep quality and brain health. One study found that brain cell activity while people sleep moves cerebrospinal fluid around the brain, cleaning it of waste products including proteins like amyloid beta and tau, which can build up and cause Alzheimer's disease. This waste is then flushed out of the brain through a "sewage" infrastructure known as the glympathic system. The other study determined that some of the debris produced by the brain's function can be removed — at least, in mice — by stimulating the neural pathways to bring about activity similar to that which occurs during sleep.

“What we found is that when we turn on this sensory stimulation, there is an increase in the [cerebrospinal fluid] movement into the brain,” Mitch Murdock, a doctoral student at the Massachusetts Institute of Technology (MIT) and lead author of the study, told Salon at the time.

If this trio of 2024 studies is any indication, the scientific world is rapidly discovering that brain health is not predetermined by our genes but very much controllable through human actions. While it will take time to develop the technology necessary to cure Alzheimer's and other brain diseases, the recent research at least improves our collective understanding of these conditions – and, therefore, what people can do to prevent them.

"While such factors as greater educational achievement and medical management of vascular risk factors may explain part of [why dementia rates have declined recently], early life environmental differences also likely contribute," the authors said.