Over millennia, the size of the human brain fluctuates, but recent research found a significant increase in brain size between individuals born in the 1930s and those born in the 1970s. This increased brain size—observable in volume, surface areas and structures like the hippocampus—could be attributable to improved health outcomes and educational possibilities. This new study was conducted by UC Davis Health researchers, and the results were recently published in JAMA Neurology.
Throughout human biology, which is measured in millions of years, some scientists have proposed that the human brain is shrinking, motivated in part by the understanding that human bodies are shrinking in size. However, recent studies indicate that the volume of the brain increased over short timelines, or biological changes across generations. Even while it might not seem like much, that might serve as a useful biological resistance against dementia.
There has been a decrease in the incidence, or quantity, of new instances of dementia in recent decades, according to many major population surveys conducted in the United States and Great Britain. One of them is the Framingham Heart Study, which was established in 1948 and has been gathering data from residents of Framingham, Massachusetts.
A 2016 study by UT Health San Antonio neurologist Sudha Seshadri and colleagues showed that although the incidence of dementia had decreased since the late 1970s, the prevalence—or the overall number of cases—had grown. Seshadri remarks, “That was a piece of hopeful news.”
“It suggested that over 30 years, the average age at which somebody became symptomatic had gone up.”
The team was left puzzled by these results: What was the reason for this lower risk of dementia? Even if throughout the years, the cardiovascular well-being of the Framingham residents and their offspring—which affects the likelihood of acquiring dementia—had also improved, this could not account for the whole reduction. Furthermore, the impact only showed up in those with a high school education, which suggested—according to Seshadri—that early life alterations might be the cause of increased resistance against dementia.
The scientists chose to look at whether the Framingham citizens’ brains showed signs of these changes because they reasoned that larger brains would result from improved health throughout development. The use of magnetic resonance imaging (MRI) for brain scans by the researchers had only started in 1999. Thus, the group looked at a subgroup of 3,226 people who were born between the 1930s and the 1970s and who had had an MRI between the ages of 45 and 74.
The team’s examination of the MRI data showed that various metrics of brain size had grown in younger generations. The intracranial volume, or brain volume, calculated by measuring the amount of space within the skull increased by 6.6% from 1236 millilitres in 1930s births to 1317 millilitres in 1970s births.
Additionally, the hippocampus, a region essential for memory processing, as well as the white matter had enlarged in volume. The outer layer of the brain, known as the cortex, had grown in surface area as well, but its thickness had decreased by almost 20%.
“Larger brain structures like those observed in our study may reflect improved brain development and improved brain health,” as per UC Davis’s Charles DeCarli, first author of the study.
“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.”
The issue remains as to how brain development across generations appears in different populations. Since most of the Framingham research participants were non-Hispanic White, healthy and well-educated, it is unclear how living in less affluent conditions affects brain size.
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