Sex beyond the genitalia: The human brain mosaic

Abstract

Whereas a categorical difference in the genitals has always been acknowledged, the question of how far these categories extend into human biology is still not resolved. Documented sex/gender differences in the brain are often taken as support of a sexually dimorphic view of human brains ("female brain" or "male brain"). However, such a distinction would be possible only if sex/gender differences in brain features were highly dimorphic (i.e., little overlap between the forms of these features in males and females) and internally consistent (i.e., a brain has only "male" or only "female" features). Here, analysis of MRIs of more than 1,400 human brains from four datasets reveals extensive overlap between the distributions of females and males for all gray matter, white matter, and connections assessed. Moreover, analyses of internal consistency reveal that brains with features that are consistently at one end of the "maleness-femaleness" continuum are rare. Rather, most brains are comprised of unique "mosaics" of features, some more common in females compared with males, some more common in males compared with females, and some common in both females and males. Our findings are robust across sample, age, type of MRI, and method of analysis. These findings are corroborated by a similar analysis of personality traits, attitudes, interests, and behaviors of more than 5,500 individuals, which reveals that internal consistency is extremely rare. Our study demonstrates that, although there are sex/gender differences in the brain, human brains do not belong to one of two distinct categories: male brain/female brain.gender differences | sex differences | brain structure | brain connectivity | behavior T he question of whether males and females form two distinct categories has attracted thinkers from ancient times to this day. Whereas a categorical difference in the genitals has always been acknowledged, the question of how far these categories extend into human biology is still not resolved (for a historical overview, see refs. 1 and 2). Documented sex/gender* differences in the brain are often taken as support of a sexually dimorphic view of human brains ("female brain" vs. "male brain"), and consequently, of a sexually dimorphic view of human behavior, cognition, personality, attitudes, and other gender characteristics (3). Joel (4, 5) has recently argued that the existence of sex/gender differences in the brain is not sufficient to conclude that human brains belong to two distinct categories. Rather, such a distinction requires the fulfillment of two conditions: one, the form of the elements that show sex/gender differences should be dimorphic, that is, with little overlap between the forms of the elements in males and females. Two, there should be a high degree of internal consistency in the form of the different elements of a single brain (e.g., all elements have the "male" form).Previous criticisms of the dichotomous view of human brain have focused on the fact that most sex/gender differences are nondimorphic popul...

Beyond sex differences: new approaches for thinking about variation in brain structure and function

“…How does this exercise apply to human brains, for which hundreds of regions have been analysed for sex differences in size, volume or other characteristics? Joel and colleagues [55] recently attempted to answer exactly this question using magnetic resonance imaging (MRI). They analysed volume, cortical thickness, diffusion anisotropy or connectivity in over 1400 human brains from four datasets.…”

“…They found that regardless of the sample, age, type of magnetic resonance imaging, method of analysis, and exact definition of the 'male-end' and 'female-end' zones, brains that had at least one region with a 'male-end' score and one region with a 'female-end' score (a condition they have termed substantial variability) were more prevalent than brains that had only 'male-end' or only 'female-end' scores. For example, defining the 'male-end' and 'female-end' zones as the scores of the 33% most extreme males and females, respectively, between 23 and 53% of brains (depending on the sample) had at least one region with a 'male-end' score and one region with a 'female-end' score, whereas the percentage of brains with all 'male-end' or all 'female-end' scores was between 0 and 8% [55].…”

Multifaceted origins of sex differences in the brain

“…Because the majority of sex differences in the brain are established early by gonadal steroids that differ in males and females, and because the brain resides in a body that is either male or female, there is an implicit, even inherent bias, that brains are male versus female. In this issue, Joel and Fausto-Sterling [8] challenge this view and cite empirical evidence based on MRI that humans are a single heterogeneous population when considering the brain [9]. Multifaceted origins of sex differences in the brain.…”

A representação binária do cérebro “feminino” e “masculino” na ciência e nos meios de comunicação

“…Esto se debe a que factores in utero y ambientales afectan la estructura del cerebro, por lo que algunos autores proponen que más que un dirmorfismo, existe un multimorfismo (Joel, 2011). Por otro lado, algunos visualizan ese multimorfismo como un 'continuum' entre lo "masculino" y lo "femenino" (Ainsworth, 2015), mientras que otros proponen que existe un 'mosaico' heterogéneo con características cerebrales "masculinas" y "femeninas" dentro de un mismo individuo (Joel, 2011;Joel et al, 2015). ¿Nacemos pre-programados con un tipo de cerebro?…”

“…Isso ocorre porque fatores no útero e ambientais afetam a estrutura do cérebro, por isso alguns autores propõem que, mais do que um dimorfismo, há uma multimorfismo (Joel, 2011). Além disso, alguns consideram esse multimorfismo como um continuum entre o "masculino" e o "feminino" (Ainsworth, 2015), enquanto outros sugerem que há um "mosaico" heterogêneo com características cerebrais "masculinas" e "femininas" no mesmo indivíduo (Joel, 2011, Joel et al, 2015. parte da engrenagem política e social que busca conceituar de forma determinista as diferenças entre homens e mulheres.…”