Sex Differences in Verbal Fluency during Adolescence: A Functional Magnetic Resonance Imaging Study in Gender Dysphoric and Control Boys and Girls

Introduction. In the literature, verbal fluency (VF) is generally described as a female-favoring task. Although it is conceivable that this sex difference only evolves during adolescence or adulthood under influence of sex steroids, this has never been investigated in young adolescents. Aim. First, to assess sex differences in VF performance and regional brain activation in adolescents. Second, to determine if untreated transsexual adolescents differ from their sex of birth with regard to VF performance and regional brain activation. Method. Twenty-five boys, 26 girls, 8 Male-to-Female transsexual adolescents (MtFs), and 14 Female-to-Male transsexual adolescents (FtMs) were tested in a cross-sectional study, while performing a phonetic and semantic VF task within an MRI scanner. Main Outcome Measures. Functional MRI response during VF task. Results. Boys and girls produced similar amounts of words, but the group MtFs produced significantly more words in the phonetic condition compared to control boys, girls, and FtMs. During the semantic condition, no differences were found. With regard to brain activity, control boys showed more activation in the right Rolandic operculum, a small area adjacent to Broca's area, compared to girls. No significant differences in brain activity were found comparing transsexual adolescents, although sub-threshold activation was found in the right Rolandic operculum indicating a trendwise increase in activation from control girls to FtMs to MtFs to control boys. Conclusions. The better performance of MtFs is consistent with our expectation that MtFs perform better on female-favoring tasks. Moreover, they produced more words than girls and FtMs. Even though a trendwise linear increase in brain activity between the four groups only approached significance, it may indicate differences in individuals with gender identity disorder compared to their birth sex. Although our findings should thus be interpreted with caution, they suggest a biological basis for both transgender groups performing in-between the two sexes.