Brain structure differences in men and women are only weakly linked to behavioral differences, according to new research that analyzed two large independent brain imaging datasets. The findings have been published in the journal Psychological Science.

The presence of sex differences in human behavior is well documented, but whether these behavioral differences are related to differences in male and female brain structure has been unclear. Previous research has indicated that sex differences in brain structure are, in fact, related to sex differences in behavior. But the new findings provide evidence that this relationship is mostly driven by brain size.

My background in neuropsychology and neuroimaging made me interested in this topic, i.e., whether sex differences in the brain are related to sex differences in behavior, explained study author Liza van Eijk, a psychology lecturer at James Cook University.

Sex differences have been found for behavior, for example, on average better visuospatial skills for males than females, and vice versa, on average better empathy skills found for females than males. Sex differences have also been found for disease and mental disorders, such as the higher prevalence for autism spectrum disorder in males vs. females, and vice versa, higher prevalence for depression in females vs. males.

In addition, several sex differences have been found in the brain, such as that male brains are on average 10-15% larger than female brains. The question that remained was whether sex differences in behavior could be related to sex differences in the brain, and if so, this relationship could provide insights for disorders that have shown sex differences.

The researchers used data from the Human Connectome Project and the Queensland Twin IMaging Study to examine individual differences in male and female brain structure. Both the Human Connectome Project and Queensland Twin IMaging Study used high-quality imaging technology to measure brain structure.

The studies also collected a variety of physical, neurocognitive, and behavioral data from participants, including but not limited to body mass index, intelligence, working memory performance, personality traits, and psychiatric symptoms. The combined sample included data from 2,153 adults.

To eliminate societal and cultural factors that could influence sex differences in behavior, van Eijk and her colleagues examined brain differences among individuals of the same-sex. In other words, they separately compared women to other women and compared men to other men.

Considering that women and men are subject to differential societal and cultural norms, we examined individual differences in brain structure along a male-female dimension, separately for each sex, van Eijk told PsyPost.

Depending on differences in genetic predispositions as well as exposure and sensitivity to sex hormones, some men will develop a more female-like brain whereas other men will develop a more male-like brain, and vice versa for women. Next, we also examined behavioral differences (such as personality and cognition) along a male-female dimension. Then, we looked at whether these brain and behavioral differences along a male-female dimension were related to one another.

The researchers found a statistically significant association between brain differences and behavioral differences, but the association disappeared after accounting for differences in brain size.

We (only) found a weak relationship between brain and behavioral sex differences, showing that brain structure differences in men and women are not strongly linked to behavioral differences and that likely many other factors play a role in behavioral sex differences, van Eijk explained. In addition, the weak relationship was mostly explained by differences in brain size, suggesting that future research examining the link between brain and behavior needs to carefully consider differences in brain size.

The study included a number of strengths, including its relatively large sample size. But, as with all research, the findings include a few caveats.

This is a correlational study, so therefore we cannot conclude anything about the direction of the effect, van Eijk told PsyPost. Further research is needed to determine whether sex (but also gender) differences in brain and behavior are the result of a common factor (e.g. masculinization of the brain early during gestation) and/or whether sex differences in the brain influence behavior, and/or vice versa, how these differences in behavior influence the brain.

In addition, it is unclear whether this relationship between sex differences in brain and behavior changes across the lifespan, in particular of interest are periods such as puberty and menopause, periods characterized by significant changes in sex hormone levels.

I believe it is important not to ignore sex differences in research, no matter how small, as examining these could provide new clues for behavior and disease showing sex differences in their prevalence or symptom expression, van Eijk added. However, it is important to acknowledge that individual differences are often larger than the sex differences observed and that sex differences found (whether in brain or behavior) do not provide any evidence for which sex is most superior.

The study, Are Sex Differences in Human Brain Structure Associated With Sex Differences in Behavior?, was authored by Liza van Eijk, Dajiang Zhu, Baptiste Couvy-Duchesne, Lachlan T. Strike, Anthony J. Lee, Narelle K. Hansell, Paul M. Thompson, Greig I. de Zubicaray, Katie L. McMahon, Margaret J. Wright, and Brendan P. Zietsch.

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Neuroscience research finds brain structure differences in men and women are not strongly linked to behavioral differences - PsyPost