09 Feb Compilation of Mosaics Explain Differences in Male and Female Brains
MedicalResearch.com Interview with:
Prof. Daphna Joel PhD
School of Psychological Sciences and Sagol School of Neuroscience
Medical Research: What is the background for this study?
Prof. Joel: The aim of the PNAS study was to test the mosaic hypothesis, published in 2011 (“Male or female? Brains are intersex”), according to which there are no “male brains” and ‘female brains” but rather brains are composed of unique mosaics of features, some more common in males and other more common in females. This hypothesis was build on animal data showing that the effects of sex on the brain may be different and even opposite under different environmental conditions (i.e., what is typical in one sex under some conditions may be typical in the other sex under other conditions).
In the Phil.Trans. paper, we use a very simple mathematical illustration to explain why the existence of sex differences is not sufficient to conclude that there are two types of brain, and how the answer to this question depends on the prevalence of ‘mosaic’ brains.
Medical Research: What are the main findings?
Prof. Joel: In the PANS study we, found differences between brains from males and brains from females, as has previously been reported. What we have done in addition, and no previous study has, is look whether brains are internally consistent, that is, have either only “male-end” features (i.e., features in the form more common in males compared to females) of only “female-end” features (i.e., features in the form more common in females compared to males). We found that internally consistent brains are much less common compared to ‘substantially variable’ brains (i.e., brains with both “female-end” and “male-end” features), and that the large majority of brains are composed of unique mosaic of “female-end”, “male-end”, and “intermediate” (i.e., common in both females and males) features. On the basis of these findings we concluded that there are no “male brains” and “female brains”.
In the Phil.Trans. paper we further suggest that human brains belong to a single highly heterogeneous population (see also Joel, 2011, Male or female? Brains are intersex), in which there may be differences between females and males in the frequencies of rare brain mosaics (e.g., brains with only “female-end” characteristics although rare in the population, are more common in females compared to males).
Medical Research: Your findings seem to contradict a lot of research done in the past.
Prof. Joel: This is not correct, because previous studies did not go beyond the level of differences in single elements. What our study does contradict is the implicit assumption that differences between females and males add up to create two types of brains, “male brains” and “female brains”
Medical Research: If brain dimorphism is a myth, how do we explain differences in behavior?
Prof. Joel: Behavior is also not dimorphic, that is, there are many ways to be human, not just two, a “male way” and a “female way”. As we show in the PNAS study, human behavior is also characterized by mosaic rather than by two distinct forms. So in fact, our results from analyzing human behavior fit nicely our results from the analysis of the human brain: they are both characterized by mosaic rather by two distinct forms.
Medical Research: In the new framework you are suggesting, how do you explain sex differences in psychopathology?
Prof. Joel: Differences between females and males in the prevalence of specific behaviors and psychopathologies (e.g., extreme physical aggression, autism) are accounted for by the existence of differences between females and males in the frequencies of rare brain mosaics (e.g., brains with only “male-end” characteristics although rare in the population, are more common in males compared to females).
- Beyond sex differences: new approaches for thinking about variation in brain structure and function.
Joel D, Fausto-Sterling A. et al
Philos Trans R Soc Lond B Biol Sci. 2016 Feb 19;371(1688). pii: 20150451. doi: 10.1098/rstb.2015.0451. Epub 2016 Feb 1. Review.
2. Sex beyond the genitalia: The human brain mosaic.
Joel D, Berman Z, Tavor I, Wexler N, Gaber O, Stein Y, Shefi N, Pool J, Urchs S, Margulies DS, Liem F, Hänggi J, Jäncke L, Assaf Y.
Prof. Daphna Joel PhD (2016). Compilation of Mosaics Explain Differences in Male and Female Brains