Our Brains are Growing

On my first day as a doctoral student at Cornell, I was intimidated by the brainy people as well as the brains.

Take the Uris Hall elevator up to Cornell's psychology department, and the first thing you'll see when the doors open is a display case exhibiting eight jars, each containing a human brain. "Those are from students who failed," said the older classmate who was showing me around. It took me a moment to realize she was joking.

What I was actually seeing were samples from the Wilder Brain Collection. Beginning in 1889, Cornell professor Burt Wilder began amassing a collection of hundreds of human brains for scientific study. The assumption then was that the physical features of the brain give rise to psychological characteristics such as intelligence, personality, and moral character. Measure the right feature, such as brain size, and you'll discover how smart the person was.  

The practice of measuring brains and characterizing their owners is alive and well. In this newsletter I'll be discussing a new study that relies on nothing more than brain size to draw important conclusions about social well-being – and offer reassurance to folks worried about cognitive decline later in life. But there were no pickled brains in this study, nor any of the flaws of pre-20th century methodology that I'll get to in a moment. Thanks to better science, better imaging technology, and modern statistics, there's been much progress in what can be gleaned from the overall size of the brain and its specific regions.

Size matters: A first pass

Peering into that display case, I was struck by how different the brains looked. E.B. Titchener, an early psychologist who coined the term "empathy", had a large, wide, relatively flat brain, vaguely reminiscent of an alien starship. In contrast, Burt Wilder's own brain, added to the collection after he passed away in 1925, is smaller and rounder.

Beginning in the late 18th century, scientists began to quantify and explore the psychological implications of such differences. Much attention was paid to brain size. The sizes of different regions, or ratios of regions, were measured, but overall volume was the most widely discussed feature. As French physician Paul Broca put it:

"In general, the brain is larger in mature adults than in the elderly, in men than in women, in eminent men than in men of mediocre talent, in superior races than in inferior races... Other things equal, there is a remarkable relationship between the development of intelligence and the volume of the brain." (Broca, 1861, quoted here).

We know now that Broca's remark starts out on the right track but quickly derails. It's true that as we get older, our brains diminish a bit in size. And yes, men do have larger brains than women – but only on average, and our brain-body ratios aren't larger. Meanwhile, there's no known connection between brain size and "eminence", nor do we see racial differences given appropriate methodology.

Broca's remark was typical of the science of the day, though the details were hotly debated. For instance, the educator Maria Montessori accepted Broca's view on eminence – she measured the heads of her students and reported a simple relationship between brain size and intellect – but she forcefully rejected the notion of the "pretended cerebral inferiority" of women. Still, the scientists who measured brain size were united in one respect: They spent much time comparing demographic groups, but the groupings were not benign.

In his 1981 classic "The Mismeasure of Man", Stephen Jay Gould describes how these scientists went to great lengths to "prove" the superiority of particular groups, including white males. Data were often massaged or selectively interpreted. At one point, discovering that comparative data favored Black people over Whites, Broca changed his measurement criteria so that Whites won the comparison. In short, ideology rather than data often served as the arbiter of scientific truth.

(I highly recommend Gould's book. Recent debates over his criticism of one particular scientist, Samuel Morton, leave out key details of his broader review.)

Does size really matter?

Broca and his younger colleague Burt Wilder were among the last of what would soon be an extinct species of researcher.

Until the end of the 19th century, there were no systematic measures of intelligence and other psychological characteristics. Rather, brain scientists had to make assumptions about group differences. This contributed to what scholars now view as a kind of circular reasoning: How do we know that larger brains imply greater intelligence? Because groups of people with larger brains are smarter. How do we know they're smarter? Because they have larger brains.

For example, the 19th century craniologists who were pouring millet, or mustard seed, or lead shot into skulls in order to calculate brain size had no way of knowing the actual intelligence of the owners of those skulls. Rather, they only knew the demographics: Each skull was "White" or "male" or "French" or whatever, and assumptions were made, without evidence, about the relative intelligence of each group. Then, a mix of unintentional as well as ideologically-driven biases ensured that the "right" skulls were largest.

By the time Burt Wilder had begun assembling his brain collection, traditional views on the correlates of brain size were starting to fall apart. Wilder himself, and others, were not duplicating purported racial differences. Gender data, after adjusting for body size, were inconsistent. And there were pesky anomalies. Criminals, being morally inferior, were presumed to have smaller brains, but exceptions kept on accumulating. (The largest brain in the Wilder collection is that of local serial killer Edward Ruloff.)

At this point, statistics entered the scene and, in retrospect, played an admirable, deeply influential role.

A statistical coup de grâce

The earliest IQ tests and other psychometric measures were developed in the late 19th and early 20th centuries. For the first time, a person could be assigned a score that precisely ranked their individual performance relative to large groups. This meant that for the first time, precise measures of brain size could be linked to precise, individual measures of intelligence, or personality, or whatever. Brain sizes could now be correlated with scores on some test, instead of being vaguely linked to stereotypes about demographic identity.

In 1902, Karl Pearson, one of the earliest and most influential statisticians, reported no correlation between the brain sizes of undergraduates and their exam scores. Data like this soon put to rest the most simplistic assumptions about brain size, including the racist and misogynistic ones mentioned earlier.

(Although we can thank statistics for helping rid neuroscience of deeply embedded prejudices, the earliest tests of intelligence and other psychological constructs were racially biased and often used in support of racist and misogynistic practices. Leading figures from Galton to Pearson, for instance, dabbled in eugenics. Applied statistics does have a lot of blood on its hands. My point here is simply that statistical contributions to psychological testing were also instrumental to the demise of simplistic ideas about the implications of brain size.)

Size matters

The overall size of the brain is still thought to be meaningfully related to psychological functioning, though not in any straightforward way.

For instance, studies do show small correlations between brain size and IQ, but the sizes of particular regions differ in importance, other characteristics of brain anatomy (e.g., synaptic connectivity) seem to be more influential, and, in any case, the correlations represent aggregate data, meaning that they tell us something about trends in large groups. When correlations are small, group data can't reveal much about specific individuals. There's no reason to feel bad if you have a small head – or proud if it happens to be large.

Moreover, intelligence is malleable to some not-yet-agreed-upon extent. Learn a new language, master a new skill, expand your social life, and you may increase your IQ and other cognitive capacities. Your brain may not grow, but studies suggest that increased synaptic density and other physical changes resulting from these activities may be the substrates of improved cognition.

This brings me to a key concept underlying the new study.

Brain reserve

Brain reserve refers to the capacity for healthy brain tissue to take on the functions of damaged areas.

This concept was first proposed in 1940 to describe an epilepsy patient whose cognitive functioning and personality were relatively normal despite surgical removal of large portions of brain. The concept is now widely discussed in other contexts. For instance, studies show that Alzheimer's disease begins to slowly damage the brain years or even decades before symptoms appear. A widely-held view is that brain reserve allows healthy parts of the brain to take over the functions of damaged parts, so that people don't experience symptoms until their reserves are "used up".

Brain reserve is also meant to account for individual differences in the effects of normal aging – i.e., people with more reserve have better cognitive functioning late in life.

Some scholars, including the researchers whose new study I'm about to share with you, equate brain reserve with brain size: The more brain tissue you have, the more your brain reserve.

This view is a direct descendant of pre-20th century investigations of brain size, in the sense of presuming that more is better. But it's a view that's free of the prejudices and circular logic of its predecessors, and most proponents acknowledge that the sheer quantity of neural stuff may not be the only foundation for characteristics such as reserve.

A new study

Brain reserve is clearly desirable, and so I'm happy to share with you a new study suggesting that we have more of it than we used to.

The study appeared this March 25th in the leading journal JAMA Neurology. Dr. Charles DeCarli (UC Davis) and colleagues analyzed data from the Framingham Heart Study (FHS), an ongoing longitudinal project that began in 1948 with 5,209 residents of Framingham, Massachusetts and currently includes the children and grandchildren of the initial sample cohort as well as other groups.

DeCarli and colleagues obtained data from 3,226 adults born between 1930 and 1970 who received an MRI some point between 1999 and 2019.

You would think that the most accurate way to measure brain size is to measure the brain itself, but this is only true if you want to know how big someone's brain is right now. If, instead, you want to know whether people born in the 1970s have larger brains than people born in the 1960s, you have to take into account their age when the measurements took place, because the brain gets smaller over time. This is a consequence of normal aging, but it can also arise from disease, injury, substance abuse, etc.

Fortunately, when we're young, brain and skull size are tightly correlated, as the skull expands to accommodate the growing brain. As we pass through our teen years, the skull reaches its maximum size. (More specifically, what's achieved is maximum intracranial volume – the amount of stuff your skull can hold.) At this point, size remains either constant or diminishes very, very slightly with age, depending on which recent studies you find most credible.

Why is this fortunate? Because if you measure a person's skull size, you get a fairly accurate estimate of their maximum brain size when they were young, before aging, disease, alcohol use, or the music of Guns N' Roses have begun to destroy any brain matter. Thus, although DeCarli and colleagues' participants ranged in age from 45 to 70 at the time of their MRI, the calculated estimates of their skull sizes offered a pretty good read on the maximum sizes of their brains earlier in life.

The graph below presents the main findings for overall size.

The y-axis is intracranial volume (ICV) in milliliters. The x-axis is decade of birth. Going from left to right, you can see a consistent upward trend. On average, our brains are growing!

Of course, people were also growing in stature during this time period. The trend shown above was observed even after controlling for height. In other words, since 1930 Americans have been getting taller, but independent of that change, our brains have been increasing in volume too. (Thanks to modern statistics for the regression procedures that allowed DeCarli and colleagues to link brain size to birth decade after controlling for variables like age, sex, and height.)

Exactly how much have our brains grown? Hard to say, because this is just one study, with a primarily White, educated sample. Other studies might yield different values. In this particular study, the average brain size (i.e., intracranial volume) of people born in the 1970s was 6.6% larger than among people born in the 1930s.

There were other findings too, such as increases in the sizes of areas or types of neural matter essential to cognition, but I want to skip ahead to some good news in how the time trends were interpreted.

The good news

Data from the FHS and other sources suggest that in recent decades, the incidence of dementia in the U.S. and Europe has been declining. (Dementia is an umbrella term for a group of symptoms that includes difficulties with memory, language, and problem solving; Alzheimer's disease is the most common cause of dementia and currently affects about 6.7 million Americans over age 65.)

To be clear, the absolute numbers of people living with Alzheimer's disease and other forms of dementia is continuing to increase. It's the incidence, or number of new cases, that's declining.

There are many proposed explanations for the decline, including healthier environments and lifestyles, better medicines, and more education. Many of these changes nurture brain growth during early development. No surprise then that as time goes on, people would tend to have bigger brains, as DeCarli and colleagues found.

The researchers propose the following chain of events: In America, beginning in the 1930s, healthier lifestyles etc. began to promote larger brains and greater brain reserve. As a result, the incidence of Alzheimer's disease and other forms of dementia has begun to decline.

DiCarli and colleagues aren't the first to propose something like this, and at this point the idea is a bit speculative (it may only be coincidental that brain growth and declines in dementia are happening at the same time), but it is an encouraging possibility.

(Given the small correlation between brain size and IQ, it's also possible that in recent decades we've been getting smarter, in a limited sense, but I'll save that possibility, known as the Flynn Effect, for another newsletter.)

Personal relevance

Assuming a relatively healthy lifestyle, recent findings, including those of the new study, imply that the chances that you or someone you care about will develop dementia by a certain age are less than they would've been in the past.

I realize that when you read that last sentence, you didn't do a fist pump and shout "yeah!". The good news from this study only sounds good if you deal in hypotheticals (if I'd been born earlier, I'd have a greater risk of dementia). But it's happy news that the incidence of dementia is declining, and that we may be increasingly acquiring "reserves" that protect against it. And there's more.

If brain reserve is nothing more than the maximum amount of brain tissue acquired when you were young, you may not be able to do anything now to bulk up, but you can avoid heavy alcohol consumption and other activities that deplete your reserves. And, you can work on increasing something known as cognitive reserve.

In analyses of what makes brains more or less resilient to aging and injury, cognitive reserve is not defined as more tissue, but rather more flexibility and adaptability in brain function. Over time, people with higher IQs and/or more education and/or greater engagement in social and other leisure activities acquire more cognitive reserve.

The nice thing about this type of reserve is that you don't max out at an early age. It can be replenished and increased anytime via learning, staying active, eating healthy, engaging with others – and, of course, following newsletters like this one.

Thank you for reading! Hopefully I've added a milliliter or two to your cognitive reserve!