And without an obvious medical benefit, Eliot thinks this type of research will simply reinforce the idea that men and women are fundamentally different, or even justify misogyny—although the authors may not intend such an outcome. This research is “far from having medical value,” she says. Instead, it can “validate the fixed, hardwired, God-given—however you want to put it—differences between the sexes, so that we can get over this idea of real equality.”
Concerns like these are one reason why sex difference research in neuroscience has attracted so much controversy. But worries about consistency have also plagued the discipline. Studies that report sex differences in the sizes of brain regions, or in how strongly some regions are connected to others, often disagree about just where those differences lie. “The longer people have been at it, the muddier it’s gotten,” Eliot says.
This inconsistency might arise from a bias among scientists in favor of reporting studies that demonstrate sex differences rather than similarities. In 2018, a group of researchers from the Meta-Research Innovation Center at Stanford found evidence that scientists are more likely to publish studies that uncover potential sex differences than those that find none. Because studies that only examine a small number of subjects are prone to false positives, the authors warn, a bias in favor of publishing those positives makes it hard to know how much that research can be trusted.
Raznahan and his team were well aware of these shortcomings, so they worked to ensure that any differences they found would reflect real patterns in brain anatomy, not the random quirks of the people in the data set. Observations that hold across a substantial number of subjects are more likely to apply to the population as a whole, which is why they relied on the Human Connectome Project’s large data set. After analyzing this data and correcting for total brain volume (just like men’s bodies are, on average, larger than women’s bodies, so too are their brains), they discovered a number of apparent differences.
Among them was a relative size advantage for men in parts of the occipital lobe (which is associated with vision) and in the amygdala and hippocampus (regions that play important roles in emotion and memory). Women, on the other hand, had more gray matter in parts of the prefrontal cortex (which is associated with decisionmaking and self-control) and the insula (which has been connected with numerous functions, including emotion and the sense of taste). These results might seem to suggest that women have an edge over men in decisionmaking and that men have better memories, but it’s impossible to extrapolate such broad conclusions from Raznahan’s results. “It could be that there’s absolutely no behavioral relevance for what we’re finding,” he says.
To begin with, it’s not clear what gray matter volume really means for brain function. The brain contains two major types of tissue: gray matter, which holds neuron cell bodies, and white matter, which connects gray matter in “tracts” and allows neurons to send signals to distant areas. They depend on each other to carry out their functions, and it’s not obvious whether having a larger volume of either one is advantageous.
“In no way is more gray matter a good thing, necessarily,” says Margaret McCarthy, a professor of pharmacology at the University of Maryland School of Medicine. “It’s just a measure that there’s a difference in the way the neurons are, how many synapses they’re making, how many there are, possibly, and stuff like that.”
Yet it’s easy for people to jump to the conclusion that size matters, and there is some evidence in favor of that notion. Two decades ago, researchers at University College London made a particularly big splash when they published a study showing that London cab drivers, who must learn how to navigate a large, disorganized city by memory alone, have much larger hippocampuses than most other people. Since the hippocampus is linked to spatial memory, this study seemed to show that the size of a brain region correlates with a particular skill. But showing that learning a skill is associated with the growth of a specific brain region does not imply that the region’s size is correlated with that ability in general. So the study doesn’t imply that simply being born with a big hippocampus means a person will have an excellent memory.