Sex differences in the human brain: a roadmap for more careful analysis and interpretation of a biological reality

Abstract

The presence, magnitude, and significance of sex differences in the human brain are hotly debated topics in the scientific community and popular media. This debate is largely fueled by studies containing strong, opposing conclusions: either little to no evidence exists for sex differences in human neuroanatomy, or there are small-to-moderate differences in the size of certain brain regions that are highly reproducible across cohorts (even after controlling for sex differences in average brain size). Our Commentary uses the specific comparison between two recent large-scale studies that adopt these opposing views—namely the review by Eliot and colleagues (2021) and the direct analysis of ~ 40k brains by Williams and colleagues (2021)—in an effort to clarify this controversy and provide a framework for conducting this research. First, we review observations that motivate research on sex differences in human neuroanatomy, including potential causes (evolutionary, genetic, and environmental) and effects (epidemiological and clinical evidence for sex-biased brain disorders). We also summarize methodological and empirical support for using structural MRI to investigate such patterns. Next, we outline how researchers focused on sex differences can better specify their study design (e.g., how sex was defined, if and how brain size was adjusted for) and results (by e.g., distinguishing sexual dimorphisms from sex differences). We then compare the different approaches available for studying sex differences across a large number of individuals: direct analysis, meta-analysis, and review. We stress that reviews do not account for methodological differences across studies, and that this variation explains many of the apparent inconsistencies reported throughout recent reviews (including the work by Eliot and colleagues). For instance, we show that amygdala volume is consistently reported as male-biased in studies with sufficient sample sizes and appropriate methods for brain size correction. In fact, comparing the results from multiple large direct analyses highlights small, highly reproducible sex differences in the volume of many brain regions (controlling for brain size). Finally, we describe best practices for the presentation and interpretation of these findings. Care in interpretation is important for all domains of science, but especially so for research on sex differences in the human brain, given the existence of broad societal gender-biases and a history of biological data being used justify sexist ideas. As such, we urge researchers to discuss their results from simultaneously scientific and anti-sexist viewpoints.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13293-022-00448-w.

Highlights

• Recent large-scale studies have reached different conclusions regarding the presence of sex differences in human neuroanatomy.

• We show that these contradictory findings are explained by different methodological choices. While multiple large direct analyses highlight small, highly reproducible sex differences, reviews do not account for methodological heterogeneity across studies (e.g., statistical power/sample size, brain size-correction methods, segmentation, region selection, participant age). This explains many of the apparent inconsistencies reported in recent reviews.

• We also summarize observations that motivate research on sex differences in human neuroanatomy (including potential causes and effects), review methodological and empirical support for using structural MRI to investigate such patterns, and outline best practices for analyzing and describing neuroanatomical sex differences.

• Finally, we argue that broader historical and societal contexts make it important to reinforce the scientific method by adopting an actively "anti-sexist" viewpoint when conducting research on sex differences in the human brain.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13293-022-00448-w.