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

Screenshot 2018-01-29 15.52.21.png
Figure via Dean III et al, 2018

By Alex Fradera

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

26 thoughts on “Sex differences in human brain structure are already apparent at one month of age”

    1. If it weren’t for the fact that you “quoted the article” I would believe that you didn’t read it. The researcher says “it would be irresponsible to draw any firm conclusions about what [the differences] might mean for function and behavior.” Differences don’t mean one is better or worse though, or that one sex is more valuable than the other. It just means they are different, which is something that shouldn’t need clarifying. This isn’t particularly surprising, but still important to investigate. I am always astounded at how many people laud diversity while lambasting the differences between men and women.

      Please stop attacking writers because you do not agree with their opinions this is not the actual paper, just a newspaper article so dont expect technical writing. And I guess a scientist that wrote a paper about neuroscience knows nothing about it and research???
      And this is mainly comments that I just copy and pasted from the great reddit discussion from the post on r/biology
      So please try to be open.

      Liked by 1 person

      1. A very interesting article which obviously draws attention to possible differences. Some comments demonstrate the needs of our changing society to instantly react without consideration or investigation and possible fear for that which they do not understand. However they are not alone as American politicians can provide many examples of this behaviour.


      2. To be fair, I think a neutral, and therefore perhaps more scientific in the traditional sense, recounting of this research would give just as many words to the consideration that the gender effects were due to society as much as biology.

        As it says: ” 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.”

        This seems an extremely fair starting point for that discussion. I would expect social feedback on biology a priori as a materialist..

        Denying gender differences i agree would be short sighted, the evidence is convincing. But the real question is whether mena and women are different inherently, or because our society makes them so.


  1. Interesting research. The fact that we may find sex differences in anatomy or function in newborns doesn’t mean that they are not socially formed from past generations. The human genome is very flexible and adapts as required for survival.


    1. @Rebecca ma’m,;, Suppose you had 170 dollars more than me. This was by {done by; caused by} nature\Biology itself. Now, the society gave me 30 dollars lesser than you. So, the overall difference (in terms of Dollars) between you and me is now 200 dollars. You now have 200 dollars more in comparison to me. Now, therefore, so, WHO has been the major contributor behind this difference? Are both Biology as well as Societal Attitudes equally responsible behind this difference of 200 dollars or whether one of these two Causes easily predominates over the other? Are both these two causes of equal value\magnitude or whether one of them happens to be the major cause (almost solely solitarily accountable for the outcome) while the other one happens to be the minor cause (nearly negligible) ?? Please answer.

      I hope that my analogy was lucid enough for you or for that matter anyone to comprehend as to what exactly I am trying to point out.


  2. ////If this difference is due entirely to sexist social structures////

    ^^ How can social order or societal actions be responsible behind BIOLOGICAL realities? This sort of a thinking is infact fundamentally flawed at its very roots itself. And it doesn’t take science to understand so,,, its extremely simple to understand.

    If indeed society’s conducts and PEOPLE’S (sexist) ATTITUDES {so–called “sexist” practices and “patriarchal” set-up} and societal structures et cetera would have been the cause behind the gender differences between males and females, then, this very same Hypothesis itself also has to imply that there used to exist no gender differences between male humans and female humans before the advent of human societies. Now, somebody please explain this simple and basic question of mine, as to, whether it is society which creates biology or whether it is just the opposite way around?! Last I had heard, it is human beings who create societies. Human beings’ bodies are made of cells and organic (bio organic) tissues and proteins and carbohydrates and all that and also such bodies are biotic objects {in stark contrast to abiotic LIFELESS objects like a block of wood or a rich deposit of iron ores or a chair or a table or a plastic bucket or the remnants of an asteroid which struck our earth or a bed of rock or a mountain peak or sand dunes or a sheet of ice or pebbles or boulders} because of certain biological and Endocrinological and Physiological functions (as also metabolism) going on within that particular volume. My body may have the same amount of volume at par with the volume of a reasonably large milk-can or container or say a vest or safe\closet or trunk or big drawer or a small shelf or a locker etc∙ but the latter is only just an abiotic object alone while I (former) am a living organism despite both of us occupying the same magnitude of volume. Why am I (the former entity) a sentient living_being while the latter is only a lifeless abiotic object? The answer is, because, no biology is applicable in case of the latter entity. Thus, it is biology which DEFINES us living beings because without this biology we would turn into a mere Lifeless volume of abiotic entity. And, human beings are also living organism. And, it is human beings which usher-in societies. So, in short, it is biology which ultimately results into society and not vice versa. Biology is the cause, society is the effect,, and not the opposite \ reverse \ converse. It is not society which dictates Biology or Biological facts or Biological occurrences or biological phenomenon. If it was indeed societal actions which were to result into Biological realities, then, someone pray answer me How did the society differentiate between males and females if there indeed was not present any biological differences between these two genders BEFORE SOCIETY’S INTERVENINGS (above–mentioned societal conducts & society’s attitudes)? Can we differentiate between two groups, unless there ALREADY is at least a few perceivable differences between those two groups BEFORE even WE BEGAN SUCH ACTIONS\conducts of ours? If society is the one who carries out such so-called gender_discriminatory deeds and out of which biological differences get arisen from, then, somebody please answer me Who had taught or tutored these people to practise such “sexist” conducts, in the first place? If I see two entities are similar, then, how is it even possible for me to distinguish between them and therefore how is possible for me to treat them differently? I can differentiate or discriminate between them ONLY IF a few initial differences {not caused by me; independent & irrespective of any doings from my side} were ALREADY extant between those two before even I started my own actions\conducts. If someone or the other hadn’t taught or tutored or advised (et cetera) the human population to exhibit such “sexist” attitudes and establish these “sexist social structures”, then, the human populations simply couldn’t AT ALL have practised these kinda conducts either. Now, whosoever had taught these lessons to our human populations, themselves are also supposed to fall under the definition of the word ‘society’ as well. So, therefore, it means, that, SOCIETY TUTORED SOCIETY (its own self) TO PRACTISE SUCH CONDUCTS and then the immediate question that automatically begs asking is that then who in the world had taught these people themselves !!! If people were taught to practise such “sexism” by their predecessors, then, the same predecessors would also have needed to have been taught such things by their own respective predecessors too. Now, thus, how (and why) did those earliest people practise such “sexist attitudes”, then? Answer is :- They crafted & pioneered such a social structure after acknowledging some important and crucial fact [acknowledging the facts and then adjusting accordingly]; they started practising such conducts because of having fathomed \ discovered some big truth, some vital piece of information. They came to know about the truths of our world\Universe\biopshere\Nature and realized as to how Physiology works and why men and men and women and women. Succintly speaking, they got to understand the truth \ facts of our Nature that males and females are widely different, and, after learning of this magnanimous truth\revelation, they made adjustments in accordance to that Truth (truth of this mortal_Universe) and which is what made populations and civilizations and Human societies so successful. Neanderthals had no gender identification amongst them and they eventually perished bcoz they acted against the Truth of Nature as opposed to Homo Sapiens who carried out necessary adjustments {called as “patriarchy” by feminists} based on those Truths\Facts. Biology dictates social order, and definitely not the other way round !! Unsubstantiatedly arguing that social order has brought into effect Biological differences between men and women, is nothing but a flimsy feeble non—cogent incoherent excuse (falsehood) given\offered by butthurt people who are afraid of the truth.

    I sincerely hope that I have been able to clarify myself.


    1. Here, in this particular comment of mine, whenever I talked about “gender differences between males and females”, I referred only to sex differences in intelligence alone. Gender differences in other aspects such as upper body strength and biomass and reproductive organs and Hormonal differences et cetera are also hugely important Truths of our human species, but, at least here, I was strictly talking {through my comment} regarding gender differences in brain merit & cognitive abilities etc∙ only.


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