Mosaic Brains? A Methodological Critique of Joel et al. (2015)

In their recent and widely publicized paper, Joel et al. (2015) made two main empirical claims. First, they claimed that the brains of individual males and females show very little internal consistency in their combination of " male-typical " and " female-typical " features, e.g.: " Our results demonstrate that even when analyses are restricted to a small number of brain regions (or connections) showing the largest sex/gender differences, internal consistency is rare and is much less common than substantial variability (i.e., being at the one end of the " maleness-femaleness " continuum on some elements and at the other end on other elements) " (p. 15472). Second, they claimed that the overlap between male and females in the distribution of anatomical traits in specific brain regions is so large that it calls into question the very idea of gender differences in brain structure, e.g.: " This extensive overlap undermines any attempt to distinguish between a " male " and a " female " form for specific brain features " (p. 15471). The first claim was based on a novel method for assessing internal consistency, which consists in the following procedure: (a) from a larger dataset, select a small number of variables (7-12 in Joel et al.'s paper) showing the largest gender differences; (b) for each individual, count the number of variables (or " features ") whose value falls in the extreme 33% of the male or female distribution; (c) classify an individual as " internally consistent " if all of the variables fall in the same category (" male-end " , " female-end " , or " intermediate "); and (d) classify an individual as showing " substantial variability " if he/she has at least one male-end variable and one female-end variable.