1/🧵 Tweetorial on our new paper, “Dump the Dimorphism: Comprehensive synthesis of brain studies finds few male-female differences beyond size.” (sciencedirect.com/science/articl…). It's big (43K words, 616 refs) so I'll cut to the chase.
2/ Surveyed hundreds of human #sexdifference studies since the dawn of MRI (structural & functional). Enough to answer the perennial question “How different are men and women’s brains?”
3/ And the answer is . . .“Hardly at all!”
4/ Brain #SexualDimorphism was coined in other animals, especially songbirds, where certain forebrain nuclei are 6X larger in males and responsible for male-only singing.
5/ Nothing comes close to this in humans. Biggest difference is the tiny (0.1 mm) hypothalamic subnucleus INAH-3, which is 1.6X larger in M and of unknown function. (Try again, @drlouann)
6/ Men have bigger bodies and average 11% larger brains. But other M/F differences are a PRODUCT OF SIZE, NOT SEX.
7/ These include: 1) gray/white matter ratio is 6% higher in women; 2) inter- vs intrahemispheric connectivity ratio also higher F>M. But both ratios vary comparably WITHIN SEX, depending on head size.
8/ Meaning the difference between the avg. man and woman is also found between small- and large-headed men (ditto among women).
9/ For specific structures, we found few reliable differences across 33 studies of subcortical volumes; amygdala only 1% larger in M. No difference in hippocampus, caudate, pallidum. 
10/ Claims of sex diffs in specific cortical volumes also widely disparate, even across 12 largest studies:
11/ Cortical thickness: MRI studies often claim F>M but postmortem finds 0.1 mm thicker in M (n.s.). Sex diffs in MRI prone to artifact from processing pipeline.
12/ Males more lateralized? Less-connected L-R? NO. Minimal sex difference in corpus callosum, anterior commissure, or hemispheric dominance. Sex accounts for mere 0.1% of variance in laterality (tandfonline.com/doi/abs/10.108…) with no behavioral relevance (tandfonline.com/doi/full/10.10…).
/13 Next, analyzed 31 studies claiming sex difference in
#connectome: results highly disparate and rarely corrected for brain size (osf.io/zdgpu/).
#connectome: results highly disparate and rarely corrected for brain size (osf.io/zdgpu/).
14/ Nonetheless, a dozen studies have used these multivariate measures to predict sex using AI. Hailed as proving #dimorphism since they can discriminate M or F with 80-90% accuracy. But this falls to 60% (vs. 50% chance) if brain size is regressed out (nature.com/articles/s4159…).
/15 Moreover, each algorithm finds different features as critical for sex discrimination, and they also perform poorly when extended to ethnically or geographically diverse populations (Sanchis-Segura et al., 2020; frontiersin.org/articles/10.33…).
/16 Meaning they have not discovered species-wide discriminators of “male brain” versus “female brain” (pubmed.ncbi.nlm.nih.gov/32141680)
17/ Finally, looked at task-based fMRI: hundreds of studies fail to identify reliable sex differences in brain activation in verbal, spatial & emotion processing. For emotion alone, 7 existing meta-analyses reveal poor consensus on sex-related activation diffs:
18/ Entire corpus of fMRI sex difference now recognized as grossly underpowered, meaning this (still widely-cited) literature is littered with false positives (journals.sagepub.com/doi/10.1177/09…; pubmed.ncbi.nlm.nih.gov/29666377/)
/19 Neuroscientists used to assume this ambiguity would be solved through technical improvements: that larger samples and better MRI pipelines would finally uncover the “real” differences between M and F brains.
/20 But the cacophony across very large studies shows there are no universal M-F differences in structures or circuits across the human species. There is no “male brain” and “female brain.”
/21 The reality is that sex-related variation in brain structure and activation is swamped by other factors: “The parsimonious conclusion is that group-level sex differences, if they exist, are buried within the greater variance of individual difference.”
/22 And yet "sexual dimorphism" lives on, invoked by experts and amateurs alike to sound more scientific, more biological: pink v. blue brains alleged to explain gender diffs in abilities, mental health, leadership @charlesmurray @DrDebraSoh @unfragilekids @MichaelGurian1
/23 Human brains ARE NOT DIMORPHIC (like the gonads), but MONOMORPHIC, like the heart, lungs and kidneys, which can be transplanted between women and men with great success.
/24 Last but important: the female brain has NOT been overlooked by researchers, contra women’s health advocates (@WomensBrainPro @dr_mosconi). Women have been equally included from the dawn of MRI.
So absence of reliable brain #sexdifference is not due the absence of data!
So absence of reliable brain #sexdifference is not due the absence of data!
/25 Many thanks to co-authors @adnanahmed_93, Hiba Khan and Julie Patel, and to the Fred B. Snite Foundation for funding.
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