Sex differences in human brain structure are already apparent at one month of age

On average, men and women differ psychologically in small but reliable ways, such as in personality, interests, and cognitive performance, but the basis of these differences is up for debate. Are they innate or due to how we’re socialised?

Neuroscientists look for traction on this question by studying sex differences in the brain, premised on the idea that these might contribute to the observed psychological differences. However, studying the brains of adults, or even teenagers, still leads to spinning wheels, because culturally produced differences will show up in the brain too. But how about one-month old infants, the subjects of a paper published in the journal Brain Structure and Function? Since birth, babies at this age have spent most of their time sleeping and suckling with limited eyesight, so profound socialisation effects aren’t going to be a factor. And yet, the new findings reveal that sex differences in a number of brain areas are already apparent.

The University of Wisconsin-Madison team led by Douglas Dean III recruited 149 expectant mothers who brought in their infants – 77 girls and 72 boys – for brain scanning one month after giving birth.

I remember doing MRI scans, and it was awfully noisy. So kudos to the infant neuroimaging community for developing a top-rate comfy environment for the little ones. They were popped into a vacuum immobilisation bag, surrounded with foam for comfort and sound insulation, and issued with ear plugs and noise cancelling headphones. These conditions allowed the scans to be performed while the infants slept, which was vital because even minor movements could have distorted the results.

Dean’s team found that the boys’ brains were 8.3 per cent bigger, in line with the sex difference in brain volume found in adults and the few other available infant studies. Also as seen in adults, male brains had relatively more white matter (connecting tissue) and female brains more grey matter, relative to total brain size.

A number of specific neural areas were larger in males, also relative to total brain volume, such as parts of the limbic system involved in emotions, including the amygdala, insula, thalamus and putamen. The researchers also found evidence for relatively larger hippocampi, an area involved in memory, which has more commonly been found to be larger in females, although not universally so. Meanwhile female brains were relatively larger in other limbic areas such as parts of the cingulate gyrus, caudate and parahippocampal gyrus, and they had a few white-matter structures that were relatively larger.

These sex differences were smaller than has been observed in adults, which suggests that maturation continues this differentiation, likely through the high volume of sex steroid receptors in these brain areas. The alternative suggestion is that the subsequent differentiation is due to socialisation, but for the forces of socialisation to work along the same lines as pre-existing biological forces would suggest that socialisation is at most a feedback loop between biology and society.

There were a lot of brain areas that differed structurally between the sexes, but it would be irresponsible to draw any firm conclusions about what they might mean for function and behaviour. For instance, what could differences in overall insula size possibly mean psychologically when the area is associated with “compassion and empathy, perception, motor control, self-awareness, cognitive functioning”, “interpersonal experience” and “psychopathology”?

The reason the new research is helpful is because it informs the interpretation of more focused studies that uncover psychological differences between the sexes. Imagine that researchers studying the treatment of anxiety find a sex-related difference in the stress response (as has happened in reality). If this difference is due entirely to sexist social structures that we’re already attempting to tear down, then the finding is not so important. But if that sex-linked difference in stress response is shown to be associated with sex-linked variation in amygdala size/structure, then the fact these anatomical differences exist so early – as shown by the current paper – makes it more plausible that the different ways men and women respond to stress is going to be hard to shake, even after social reform. Attempts to make society fairer are more likely to succeed with these facts in mind, rather than hoping they will go away.

—Investigation of brain structure in the 1-month infant

Alex Fradera (@alexfradera) is Staff Writer at BPS Research Digest