Why Brain Size Doesn’t Correlate With Intelligence

You are a child who grew up.

The inevitability of a child’s growth is both celebrated and mourned. Under normal circumstances, parents can do little but stand back and watch as shoe sizes climb, squawks become babbles become words become speech (which soon becomes back talk) and a child’s knowledge of math, to say nothing of her texting dexterity, outpaces her parents’.

For human beings, growth in childhood leads to maturity, a relative concept: mature with respect to what?, one might ask of a teenager. Biologically, growth is the destiny of all successful organisms. But that growth comes in myriad possible forms—growth from zygote to neonate to adult, growth in size or stature, growth of an entire species. Another form, growth in brain size, has long been linked to success.

A recent study in the Proceedings of the Royal Society B appears to suggest that, in a sampling of wild animals, average brain size—measured from craniums of museum specimens collected from urban and rural animals over the last century or so—is correlated with living around people. Both the city mouse (of the white-footed variety) and city vole (of the meadow variety) had larger cranial capacities than their rural cousins. Whether it was the more cerebral rodents that moved into the city, or whether the species adjusted to the novel challenges of an urban environment by growing their brains over generations, isn’t known. Regardless, headlines proclaimed: “City Mice Smarter Than Country Mice.”

Such news excites us humans, who take pride in our big brains. The notion that brain size indicates cognitive prowess is, of course, flattering for us. The further notion that cities house the bigger-brained—why, that’s hard for the urban sensibility to ignore.

What the headlines didn’t crow about was the researchers’ finding that only two of the ten investigated mammals had bigger brains in their urban variants. And the cranial capacity of two shrew species (short-tailed and masked) and two bats (little brown and big brown) grew bigger over the decades in rural, but not urban, settings.

People have long been tempted to link brain size and cognition. The intuitive notion that a “big brain” means “more intelligent” was first threatened some time ago, when we discovered animals with larger brains than ours: elephants and whales. Sure as we were of humankind’s superior intelligence, we still felt the need to prevail, so we gamely parried: Perhaps it is the brain size relative to body size that makes our brains the biggest. Though humans come out well there, too, this measure is biased toward birds and other small animals that have relatively large brains for their bodies. After more deliberation, scientists finally offered up the so-called “encephalization quotient”: brain size relative to the expected brain size in related taxa. On top: humans. Phew.

Consider, though, the strange case of that growing child. Every infant’s brain develops through a period of synaptogenesis—wanton proliferation of synapses, which are the connections between neurons—in the first year or so of life. But one could argue that it is when this intense brain growth ends that the real growth of the child qua individual begins. The next phase of brain development occurs in large part through an increase in synaptic pruning: paring of those connections that are not useful for perceiving, considering or understanding the world the child is facing. In this sense, it’s by downsizing that an individual’s brain is born.

Brain size, or the size of brain parts, can be a reasonable indicator of skill, to be sure. In individuals with sensory deprivation other sensory inputs take over the cortical area lying dormant. In the case of blindness, auditory or tactile somatosensory areas may grow in size, and hearing or touching sensitivity will improve accordingly. Dramatic as that compensatory growth may be, in the end the correlation between brain size and brain function is fraught.

Consider the humble dog, Canis familiaris. The brain of a wolf-size dog is about 30 percent smaller than that of an actual gray wolf, its ancestor. Has the dog become less smart since it went its own evolutionary way thousands of years ago? Judge for yourself: When the mere gaze from the dewy eyes of a member of this species causes you to get up from the couch, repair to the refrigerator and retrieve a hunk of cheese for your charge—well, you tell me who is smarter.

The dog is successful not because of the size of its whole brain per se, but because domestication has led to subtle brain changes with a stunning result: the ability to live in the world of people.

To the brain reading this: You may grow as you process these words. But almost certainly, your growth will not be as simple as an increase in size. Synapse that!