Fig. 2

Methodological differences across studies explain apparent inconsistencies in reported sex differences in amygdala volume. Results and design characteristics of studies on sex differences in amygdala volume, including the direct analyses by Williams and colleagues [2] and the studies collated by Eliot and colleagues [1] (N = 31). A Each point represents one study: color = detected sex-bias, size = sample size, shape = brain size correction and segmentation method combination (see legend). Inset depicts the plot excluding Williams and colleagues [2]. B Bar plot depicting the sex-bias (color) per correction and segmentation method (with the Williams et al. study isolated), scaled by the sum of the underlying study sample sizes. C Bar plot depicting the sex-bias (color) per correction and segmentation method, scaled by the underlying study counts. D Bar plot depicting the sex-bias (color) across the studies tallied by Eliot and colleagues and the study by Williams and colleagues, scaled by the sum of the underlying study sample sizes. Loss of information regarding analytical methods and sample sizes accounts for apparently inconsistent findings. Notably, the only studies that detect female-biased amygdala volumes use the proportionalization method for brain size correction (which introduces biases—see text) combined with Freesurfer segmentation. Studies that report non-significant differences tend to have smaller sample sizes (mean N across studies: ns N = 557; female-biased: N = 859; male-biased: N = 4366) or are meta-analyses of studies that used various methods. In fact, all large studies that used the covariate or VBM correction methods (N = 6 studies with sample sizes > 1000) detected male-biased amygdala volumes in their primary analyses