The authors test the effects of a mean ten months of testosterone on a mental rotation and brain region activation on a group of 21 gender dysphoric girls. The girls were also treated with puberty blockers for a mean two years before beginning testosterone, but this was not the primary study objective.
21 girls and 20 boys without gender dysphoria and of similar ages, served as the controls. The researchers tested performance in mental rotation and took brain scans in two sessions:
- after puberty suppression and before testosterone, and
- after testosterone treatment.
The study population appears to overlap with the group studied in Staphorsius et al. (2015) (similar authorship and the studies have similar methods).
At the first session, after 2-48 months of puberty suppression, the treated girls had an IQ of 100.5 -significantly (p = 0.009) lower than control girls (110.3) and control boys (113.4). It is not clear if this was an effect of the puberty blockers, the gender dysphoria, or some other factor. Mental rotation accuracy was also lower but not significant.
At the second session after the clinician withdrew puberty blockers and treated the group with testosterone for ten months, the treated group had the highest mental rotation accuracy, but this was not significant compared to the controls.
The treated girls had a more male-typical brain activation pattern at both sessions. All of the treated girls were lesbian (gynephilic), and all the control girls were heterosexual. The authors hypothesise that differences in sexuality could account for the difference in brain activation at the first session (though they could not rule out puberty blockers).
The authors propose that testosterone exposure influenced further masculinisation seen in the second session. Testosterone exposure is known to affect mental rotation accuracy and could also account for the improved mental rotation scores.
The study design does not allow not us to draw conclusions about the impact of blockers on cognitive function. The differences in IQ between the groups after puberty suppression is concerning. The study follows the same pattern as Staphorsius et al. (2015), where all participants were homosexual. A connection between homosexuality, anti-homosexuality attitudes, and gender dysphoria goes unexamined by the study authors.