MedicalResearch.com Interview with:
Prof. Daphna Joel PhD
School of Psychological Sciences and Sagol School of Neuroscience
Tel-Aviv University
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).
(more…)