Human Brains May Be Getting Bigger

Human Brains May Be Getting Bigger

Brain size in one Massachusetts community has steadily increased since the 1930s, possibly explaining why dementia is trending lower nationwide

By Diana Kwon

Laguna Design/Getty Images

Overall, people in U.S. live longer than they did a hundred years ago. The growing number of people reaching old age has meant an increased proportion are at risk of developing dementia or Alzheimer’s disease, illnesses that typically strike later in life. However, researchers have found that, in the U.S. and elsewhere, dementia risk may actually be decreasing, at least in a subset of the population. A new study provides a potential explanation for this trend: Human brains may be getting larger—and thus more resilient to degeneration—over time.

Several large population studies in countries including the U.S. and Great Britain have found that, in recent decades, the number of new cases, or incidence, of dementia has declined. Among these is the Framingham Heart Study, which has been collecting data from individuals living in Framingham, Massachusetts since 1948. Now accommodating a third generation of participants, the study includes data from more than 15,000 people.

In 2016, Sudha Seshadri, a neurologist at UT Health San Antonio and her colleagues published findings revealing that while the prevalence—the total number of people with dementia—had increased, the incidence had declined since the late 1970s. “That was a piece of hopeful news,” Seshadri says. “It suggested that over 30 years, the average age at which somebody became symptomatic had gone up.”

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These findings left the team wondering: What was the cause of this reduced dementia risk? While the cardiovascular health of the Framingham residents and their descendants—which can influence the chances of developing dementia—had also improved over the decades, this alone could not fully explain the decline. On top of that, the effect only appeared in people who had obtained a high school diploma, which, according to Seshadri, pointed to the possibility that greater resilience against dementia may result from changes that occur in early life.

Hypothesizing that better health during development would lead to bigger brains, the team decided to investigate whether such changes were apparent in the Framingham residents’ brains. Brain scans, which the researchers conducted using magnetic resonance imaging (MRI), had only begun in 1999. So the team examined a subset of 3,226 individuals, born between the 1930s and the 1970s, who had undergone an MRI when they were between 45 and 74 years old.

The team’s analysis of the MRI data, which was published in March in JAMA Neurology, revealed that, in younger generations, several measures of brain size had increased. Brain volume, which was determined by measuring the space within the skull (also known as intracranial volume), had grown by 6.6 percent, from 1236 milliliters in those born in the 1930s to 1317 milliliters in those born in the 1970s. The volume of both the white matter and the hippocampus—the latter a region key to processing memory—had also become larger. The surface area of the cortex (the outer layer of the brain) had also increased, but the thickness of the cortex had shrunk by about 20 percent.

Seeing a decrease in cortical thickness was a bit surprising at first, says Charles DeCarli, a neurologist at the University of California Davis Medical Center and first author of the study. But these findings make sense when considering that, as the brain gets larger, it also becomes more convoluted—a process known as gyrification, which stretches out the surface area of the cortex. A potential consequence of this change might be that the brains of those in later generations are not only getting bigger, but they are also getting more interconnected, DeCarli speculates. An increase in white matter, containing the connective fibers of brain cells, also provides evidence for this hypothesis, he adds.

Brain size reaches its peak in adulthood, and intracranial volume remains stable across the lifespan (except in rare cases associated with bone disease that distorts the shape of the skull). Thus, the authors note that larger brain volume may reflect the influence of environmental factors, such as education and overall health, during development. According to Seshadri, determining what exactly is accounting for the changes in brain size is an important next step to be addressed. There may also be environmental factors that cause brains to shrink—a 2023 study reported that, over that past 50,000 years, human brains tended to be smaller during warmer periods than cooler ones, suggesting that climate change may produce this effect. Since the beginning of the human family tree, which emerged some 7 million years ago, our brains have tripled in size.

“This is a nice study, but it needs more validation,” says Prashanthi Vemuri, a neuroimaging scientist at the Mayo Clinic who wrote an editorial accompanying this study. Replicating these findings in a larger sample and conducting longitudinal studies to examine how brain size changes across the lifespan, will make these findings more convincing, she adds.

Another open question is what brain growth across generations looks like in other populations. The majority of the participants in the Framingham study were healthy, well-educated, and non-Hispanic Whites—so the effect on brain size of living in less privileged circumstances remains to be seen. “These findings need to be explored in other more racially, ethnically, geographically, and socioeconomically diverse populations to see if this holds true,” Seshadri says.

Seshadri and DeCarli note that they and their colleagues are planning to address some of these remaining question in future studies. In the meantime, they note that this work emphasizes that it is crucial to consider both early adulthood and childhood when thinking about dementia prevention.

Carol Brayne, a neuroscientist and epidemiologist at The University of Cambridge who was not involved in the study, agrees. “The Framingham study has been phenomenally important in understanding brain health and how it evolves across the age groups included,” she says. This particular study adds to the growing body of evidence pointing to the importance of addressing risk and resilience factors for dementia across the lifespan, shesays.

“Across the globe, there are children who are going to be more likely—if they survive—to develop dementia because they’ve had sub-optimal life courses,” Brayne says. “The inequalities that we are observing can have major impacts. This is something that policy makers need to pay attention to.” This is something “policymakers need to pay attention to.”

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