Human BNST volume is not simply sexually dimorphic

‘The bed nucleus of the stria terminalis (BNST) is a sexually dimorphic basal forebrain region’, is a claim prevalent across rodent and human neuroscience research, with particular emphasis on its substantially larger size in males. Despite the pervasiveness of this claim, with potential implications for understanding sex differences in anxiety and substance use disorders, inspection of prior literature reveals a complex and nuanced picture. Direct evidence for larger male BNST size in humans comes solely from a handful of mostly small-scale post-mortem studies, which show either no, moderate, or very large differences, therefore indicating the need for a larger systematic investigation. Addressing this, we developed a novel 3T T1-weighted (T1w) manual segmentation protocol of the BNST, which was applied to ultra-high resolution T1w structural MRI data in 170 young human adults. Using a Bayesian modelling approach, taking into account existing post-mortem data, and controlling for total brain volume, age, and sibship, we find little evidence for total BNST volume differences between males and females. We recommend that researchers exercise caution when reporting evidence of BNST sexual dimorphism, particularly when translating findings from rodent models in which the BNST may play a different, olfaction-focused, role.