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I don't mean to cause offence or suggest than men and women shouldn't be of the same intelligence — that's not something I'm here to dispute. What I want to understand is how this is possible given the vastly lower average synaptic / neurone count and weight in the average female vs male brain, in terms of both the overall brain and the neocortex.

Are there some systems that the female brain employs to compensate for a lower number of neurons, and if so is it dependent to the hormone estrogen or is it something inherent to the female brain? Is it just the case that the male brain has vast redundancies, if so why does brain weight correlate with IQ if it is redundant? Or perhaps it's the discrepancies in synapse count do not extend to areas that are most closely linked with higher order thinking. This is the explanation that I am leaning to, and if someone could explain which areas these are and link to a study that shows that the male vs female average in these areas are the same, then that would be greatly appreciated (I'm not looking for areas that are the same size adjusted for total body weight, I'm looking for areas with the absolute same number of synapses/neurones).

Studies:

Female brains on average lighter than male brains: https://pubmed.ncbi.nlm.nih.gov/8072950/

Female brains on average have lower neocortical neurone count than male brains: https://pubmed.ncbi.nlm.nih.gov/9215725/#:~:text=The%20average%20numbers%20of%20neocortical,brains%2C%20a%2016%25%20difference.&text=Sex%20and%20age%20were%20the,no%20influence%20on%20neuron%20number.

Female brains have on average a lower cortical synaptic density: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2567215/

Context:

I am transgender (and taking estrogen) and have read that taking estrogen can move brain volume and weight closer to average "female proportions" to quote the study, and that this does not appear to negatively affect cognition. So, this is a question that is hugely important to me and does not come from a place of mal-intent.

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    $\begingroup$ Note the last study has a sample size of 8 meaning it is completely worthless. honestly trying to establish something for humanity as a whole with sample size of 8 makes the study a waste of paper. $\endgroup$
    – John
    Apr 2, 2021 at 6:29

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In brief, so far as we can tell size matters far less than brain architecture.

Consider that humans have only about half as many cortical neurons as a number of whales, yet are clearly far more intelligent.

While we are still far from a complete understanding of the details of how intelligence actually works, neuron count is only part of the story.

For humans in particular, there is also a selection argument that would lead us to expect that relatively small differences in neuron count (e.g., between male and female average) are not particularly significant. General intelligence (not "IQ") appears to be a critical aspect of human selection, and thus under significant evolutionary pressure for both sexes. If a small adjustment in neuron count made a significant different in intelligence, cranial and cortical scale is a relatively simple morphogenetic adjustment, and we would expect both sexes to scale until they hit a more similar scaling boundary. Since that has not happened, it is reasonable to conclude that neuron count is at least not a major factor in intelligence.

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  • $\begingroup$ If the correlation between general intelligence and brain size is so small that males and females have the same average general intelligence despite differing brain sizes, then why is the correlation large enough to even be observed? Surely if global brain size correlates even slightly with general intelligence that we would see a very small difference between the sexes? $\endgroup$
    – tay_100
    Apr 2, 2021 at 1:11
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    $\begingroup$ So far as I have been able to find, it's not clear if there is any actual relationship. There are too many other factors that could be causing the correlation. For example, brain size also appears linked with nutrition, which is linked with wealth, which is definitely linked with decreased disease burden, increased educational opportunity, etc. If there's any effect, it's small, which combined with the scientific click-bait factor makes it a very messy subject to try to study. $\endgroup$
    – jakebeal
    Apr 2, 2021 at 1:22

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