Objective: Attention-deficit/hyperactivity disorder (ADHD) is more common in males than females, yet little is known about why this is the case. We aimed to test for the existence of a ‘female protective effect’ against ADHD traits in two European twin samples. Specifically, we aimed to test whether cotwins of females displaying extreme ADHD traits would display more ADHD traits than co-twins of males displaying extreme ADHD traits. Method: Parents of approximately 7,000 pairs of non-identical twins in Sweden, and around 4,000 pairs of twins in England and Wales, completed dimensional assessments of ADHD traits. Probands were selected on the basis of scoring within the highest 10% of the distribution in each sample. Dimensional scores of co-twins of probands, as well as the categorical recurrence rate, were investigated by proband sex. Results: Co-twins of female probands displayed higher mean ADHD trait scores (􀝔̅=0.62-0.79) than co-twins of male probands (􀝔̅=0.38-0.55) in both samples. This trend was significant in the Swedish sample (p