Sex differences in brain organization and functions are the last frontier in this line of research, and their study is shaping up to be a ruckus. The results will eventually resolve the issue of the relative contributions of biological and cultural influences on sex differences. The associated scientific and social policy stakes are high and far-reaching. Minimal sex differences in brain organization and functions would help to affirm the importance of socially imposed gender roles and support the use of policy interventions to reduce sex differences in important social outcomes. Substantive biological contributions, on the other hand, would raise consequential questions about the wisdom of such interventions.
For example, the European Union recently conducted a large-scale analysis of sex differences in adolescents’ aspirations for entering science, technology, engineering, and mathematics (STEM) fields. Women are under-represented in many of these fields (e.g., engineering), and this is often framed as an imbalance that must be addressed. The EU study did not find a clear reason for the imbalance, but nonetheless recommended EU-wide policy and funding initiatives to increase girls’ interest in these fields. Such initiatives may be successful if sex differences are driven by gender role expectations and stereotypes. However, the money for these initiatives would be better spent elsewhere (or not spent at all) if biologically influenced differences in occupational preferences and cognitive strengths (e.g., in spatial abilities) substantively contribute to the sex differences in STEM and in other areas of life.
Evidence for biological contributions to sex differences are typically ignored or downplayed in such policy documents, which might be justified if neuroscience research reveals that physical sex differences are scraped clean in the brain. Indeed, in a review of brain-imaging studies, Elliot and colleagues concluded that sex differences in the brain are trivial and unimportant. To be sure, findings in this area are often contradictory and no clear consensus has emerged on the nature and extent of neural sex differences. However, DeCasien and colleagues have argued that these contradictions are related in part to the use of different technologies (e.g., to precisely locate brain regions) across studies. Large-scale studies that use the same technology and control for confounds (e.g., brain size) reveal consistent and substantive patterns of sex differences in the brain.
The largest of these studies was recently conducted by Williams and colleagues and included more than 40,000 people. It controlled for factors that could distort the findings (i.e., brain size), and replicated sex differences found in another large-scale study (see Figures S8, S10, S12). Adjustments for overall brain size revealed numerous areas that are relatively larger in women or men. Given the cost of building and maintaining the human brain, we’d expect evolution to downsize these areas unless they provided important advantages in terms of attending to and processing specific features of the social and physical world.
In Williams and colleagues’ study, the sex differences in specific brain regions were small to moderate in size, but many were part of the same network. Multiple enhancements in the same network will result in larger and functionally important sex differences. Williams and colleagues did not discuss the implications of their findings for understanding sex differences in behavior and cognition, but the implications are there. To explore these, I took the ten areas that showed the largest advantages (in cortical volume, area, and/or thickness) for women and men, reverse-engineered them in terms of their basic functions, and then integrated these with currently known sex differences. The results provide a glimpse into differences in women’s and men’s orientations to the social and physical world.
Disproportionately (controlling for brain size) larger brain areas in women than men support language, social cognition (sometimes called emotional intelligence), emotional processing and reactivity, and contextual and spatial memory, among others.
The language-related areas are found in the left and right hemispheres and include areas that support the processing of basic language sounds, language production and fluency, the syntax of utterances (spoken sentences), verbal memory, and language perception, such as the identification of individuals based on their voice. Most of these areas can be divided into subareas that process somewhat different types of information. Williams and colleagues’ findings should therefore be interpreted with caution because sex differences weren’t assessed in these subareas.
Nevertheless, the finding that women have relative advantages in multiple brain areas that compose the language network is telling and consistent with autopsy studies; the latter provide a fine-grain analysis of specific sex differences. Harasty and colleagues, for instance, found that areas that contribute to processing language sounds and language production are proportionately (controlling brain size) larger by 20–30 percent in women than men. Witelson and colleagues found that at least one of these areas is more densely packed with neurons in women’s than men’s brains. The implication is that the language system is relatively more important for women than are some other systems (e.g., for object processing, below).
The language system is integrated with brain areas that support the processing of social information. These include several areas that support the recognition of faces and the processing of emotions conveyed through facial expressions and body language, as well as sensitivity to the direction of gazes and the location of sounds, such as utterances. Many of these areas are also integrated with the default mode network that provides “a self-centered predictive model of the world.” The network contributes to feelings of agency, self-awareness, personal memories, thinking about the world in self-referential ways (see here also), and theory of mind (mentally and emotionally putting oneself in another’s shoes).
Again, all these areas have multiple functions, so these descriptions should be interpreted with caution. Nevertheless, they converge on a few themes and are consistent with behavioral studies. These include a sex difference in empathy found across primate species, sensitivity to facial expressions and other social cues, emotional intensity, ruminating about negative experiences, and in general default mode functions. The latter appear to contribute to sex differences in depression-related rumination, among other things.
Women also have relative advantages in some areas associated with spatial abilities (e.g., areas of the parahippocampal gyrus), which typically favor men. These areas also support non-spatial abilities including memory for personal experiences and context memory—memory for things that typically go together. Severalstudies suggest that men may engage this region more efficiently during spatial navigation than women, but in any case, the integration of spatial information into context memory is consistent with women’s advantage in object location memory; this is thought to be related to an evolutionary history of foraging.
The gist is that, relative to men’s brains, women’s brains have enhanced language abilities and are more attuned to interpersonal social information, their own emotional experiences, personal memories, and perhaps are biased to interpret events in self-referential ways. These enhancements meld with the argument I advanced in my book, Male, Female: The Evolution of Human Sex Differences—that women have an evolved motivation to develop and maintain a small but cohesive network of close social relationships that provides them and their children with social and emotional support, as well as access to culturally important resources and a safe environment.
Women’s greater gregariousness, warmth, and expressiveness (e.g., smiling) coalesce into a style that facilitates close relationships and conveys that they are not a threat (increasing safety). In the context of these relationships, girls and women are more reciprocal (equality of outcomes) and disclose more personal information than do boys and men. Enhancements in social-cognitive brain areas would facilitate competence in the context of the associated interpersonal interactions, in keeping with prior studies showing a sex difference in sensitivity to the social-emotional cues of their partner.
Enhanced emotional sensitivity provides advantages in many contexts, although it also increases false alarm rates (i.e., overreacting). Sensitivity is important for detecting and reacting to sometimes subtle social information conveyed in interactions (e.g., a fleeting facial expression). When combined with extensive knowledge of the other person, this social sensitivity supports theory of mind inferences about the source of the distress and helps to produce a supportive response.
Enhanced sensitivity to social cues, nuanced language competencies, and emotional sensitivity also contribute to the use and detection of relational aggression. This involves the manipulation of social information to undermine the reputation of competitors and to disrupt their social support network or push them out of the local social circle. The strategy includes gossiping in ways that provide unpleasant information about a competitor but in ways that can be plausibly denied as aggressive (“I’m really worried about Sally because she’s drinking a lot and sleeping with a lot of guys. We need to help her…”). The subtlety requires enhanced sensitivity to social cues and an enhanced emotional reactivity to them that motivates further examination and development of a counter-strategy. The examination often involves a rehashing and discussion of the episode with a close friend and sometimes ruminating about it.
So, we might imagine women being at the center of a self-created social network (family, friends, romantic partners) in which they know a lot about the individuals in the network and are emotionally connected to all of them. The knowledge and emotional connection provides a barometer that monitors the well-being of others in the network, and when things go awry, it triggers an empathic emotional reaction that motivates a supportive intervention. The sex difference in the self-referential default mode network seems to contradict women’s communal orientation to life, but it doesn’t really. Interpreting events in self-referential ways is consistent with the sex difference in concern for safety, but this includes the safety and well-being of others in their network; under threat, women not only think of their own safety, but also that of friends and family.
Disproportionately larger brain areas in men than women support the processing of object features and mechanical knowledge, attending to large-scale space and spatial navigation, attentional control and decision-making in risky situations, and some aspects of the social world. Again, all the associated areas have multiple subareas and functions, but they converge on several common themes.
Numerous areas that support the processing of object features (e.g., shapes), how to use objects as tools (mechanical reasoning), as well as a general conceptual understanding of tools are disproportionately larger in men than women. These range from low-level visual processes to an integrated system that supports manipulating objects as they are being used as tools, and conceptually understanding object-tool relations. These findings are consistent with a male advantage on tasks that require the mental rotation of objects, which is thought to contribute to tool use. Chen and Siegler found that two-year-old boys have a better intuitive understanding of how objects can be used as tools than do same-age girls, and that there is a large sex difference in mechanical reasoning. These findings are also consistent with men being the primary tool makers in traditional contexts (and presumably during human evolution), and with aspects of the extreme male brain theory of autism.
Men’s advantage in various visuospatial abilities (except object location memory) is well-documented, and the areas identified in Williams and colleagues’ study are especially important for scene processing and memory, navigation in large-scale space, and generating a birds-eye view of the environment. These are in keeping with moderate to large sex differences in actual dead-reckoning navigation, map reading and drawing, and enhanced memory for routes, among related abilities. Sex differences in spatial navigation and memory are found in species where males have larger travel ranges than females and this is the case for people in traditional and modern contexts, so nothing surprising here.
The attentional and social processing regions identified by Williams and colleagues contribute to attentional and inhibitory control, decision-making under risky or uncertain conditions, as well as social reasoning, a cognitive mapping of social hierarchies, and sensitivity to angry facial expressions. These findings are consistent with more intense male-on-male than female-on-female aggression, which in turn is due to a much stronger relation between socialstatus and reproductive outcomes in men than women. The findings are also in keeping with the sex differences in status-related concerns and striving (within groups and between them), as well as status-driven risk-taking. These brain areas may also contribute to the sex difference in a cold-cognition approach to social strategizing, but this remains to be determined.
In any case, the implication is that men’s brains have features that will result in an enhanced attunement to certain features of the physical world. These include attending to and processing information about objects, their properties (e.g., shape), and how they might be used in practical ways. These features also include numerous areas that facilitate an attentional focus on large-scale space and forming mental representations of this space which provides advantages in knowing where they are and how to get where they want to go. The processing of social information appears to be biased toward forming abstract representations of social hierarchies and social decision-making associated with threats or other risks. Again, all the areas highlighted here have multiple functions, but they have a few common themes that are consistent with prior research and that distinguish them from the themes that emerge for women.
It will likely be decades before we fully understand brain organization, its functions, and associated sex differences. Even so, and despite spirited arguments that any such differences are trivial, advances continue and are revealing systematic sex differences in the brain. Each of these differences is small but when many of them are related to the same competence (e.g., object processing, mechanical reasoning) the end result is a much larger and important sex difference.
One argument is that any sex differences in the brain are due to differences in experiences that are driven by socially imposed roles. Indeed, experience is necessary for the full development of many brain systems, but many of these experiences are self-initiated, not simply the result of social pressures. Boys and girls engage in different types of play activities and form different types of social relationships that very likely tweak aspects of brain development, but these sex-typical activity differences are more strongly related to early hormone exposure than to social edicts.
Women’s under-representation in STEM is primarily in fields that involve working with things, such as engineering and computer science. These inequalities are related to the well-documented sex difference in interest in people (favoring women) and things (favoring men), and associated occupational aspirations and choices. The areas identified in Williams and colleagues’ study coalesce around these same themes, with women’s brains having features that would enhance sensitivity to interpersonal social cues and men’s having features that orient them to aspects of the physical world. The latter includes enhancement of areas that support mechanical reasoning and spatial abilities, both of which contribute to interest in and success in things-related STEM fields.
Given this, the sex differences in occupational segregation in STEM and in other areas of life are not surprising and not likely to change even with continent-wide interventions. Pushing the equal-outcomes agenda in STEM and in other socially valuable areas provides feel-good experiences for gender activists, a self-serving diversion of the public’s money, and lots of bureaucratic jobs. But in many cases, it will not have the intended effects and it is therefore not a good use of these resources.