"I am a boy: I learn better when my body is in motion; I have more serotonin in my body than girls, so it takes longer for me to calm down when I am mad; my strengths are my aptitude for visual spatial memory and abstract reasoning; the learning environment at school does not fit my needs very well.

I am a girl: My strengths are my emotional memory and detecting social signals; the strong wiring between my left and right brain makes me good at multitasking; I am well able to express my feelings; the school environment fit my needs as I can easily sit still."

Ongoing debate
These statements are taken from a poster hanging in a teachers’ bathroom at a primary school in the Netherlands. The poster illustrates the ongoing debate on why boys perform less well than girls at school. A commonly heard argument in this debate is that male brains are wired or develop differently. However, scientists have been puzzled for years about where, when, and how male and female brains differ. Do boys and girls really differ such that the male brain could have been designed on Mars and the female brain on Venus? Thus far group level differences have shown great inconsistencies, and no specific set of brain features has been detected that fully distinguish male and female brains. Our recently published study provides novel insights by showing that we should have another take on sex differences in the brain and move beyond studying mean differences.

Large overlap
One reason to do so is that solely focusing on mean group differences ignores the large amount of overlap (see here for an in-depth discussion on this issue). Most brain and cognitive measures show a larger overlap than difference between boys and girls, as indicated by small effect sizes. Even in domains where the largest effect sizes were observed, such as in a 3D mental rotation, the majority of girls perform similarly to the majority of boys (>80% overlap). On top of that, the differences within a group may be larger than between the groups: for example, the difference in abstract reasoning between any two randomly selected boys is larger than the mean difference between boys and girls.

Differences in variability
Importantly, recent studies claim that greater variability between boys than between girls is a fundamental aspect of sex differences in humans. One study showed, for example, that even in the absence of mean differences more males than females scored in both the 5% highest and the 5% lowest range for math and reading performance. Our study was the first to extend these findings to the brain: we showed that larger male variability is observed in the size of a number of brain structures, including the cortex and subcortical structures involved in reward-processing and memory functioning. This means that the variability between girls’ brains is smaller than the variability between boys’ brains; in other words, girls show a regression to the mean, while a larger number of boys show relatively extremely large or extremely small brain structures. Our data make clear that dimorphism, the existence of two distinct forms, is not an accurate way to characterize differences between male and female brains. These results have important implications for the interpretation of sex differences in brain structure and cognition. First, the difference in variability between boys and girls is problematic for studies using small samples. Difference in variability between groups may result in false positive mean group differences, as illustrated in a recent methodological publication. This may in part explain the mixed findings in studies on sex differences in regional brain volumes. Another implication is that the larger number of boys at both the upper and lower extremities may result in stereotyping; the observation that the majority of boys and girls show large overlap is often ignored.

Effects of sex
How should we then interpret sex differences? First, rather than talking about “sex differences” it may be more accurate to talk about “effects of sex”, just as genes and experience may also have an effect on shaping the brain. Thus, on an individual level the effect of training (experience) may be larger than the effect of being a boy. Importantly, teaching students about how plastic their brains are has been shown to improve their study performance. Moreover, framing believes prior to a test showed to completely remove sex effects, even for a 3D mental rotation task. When women were led to believe they were better than men, women’s performance reached men’s. We should be aware of the strong effects stereotype threats may have on study performance. To conclude, when putting brain research into practice, it is important to take into account that brains of boys and girls are more similar than different and not opposite ends of a single continuum. In addition, we should be aware that the larger variability in boys than girls may contribute to misconceptions and stereotyping. I suggest we revise those posters above and educate students about their brain’s opportunities rather than highlighting their limitations on the basis of their sex.