Figure 2. The set of test faces.

From left to right: female (0% male), relatively female (20% male), neutral (50% male), relatively male (80% male) and male (100% male). In a pilot study we asked participants (n = 15) to rate the gender of 9 different faces ranging from absolutely female to absolutely male (10 repetitions per face and presented in random order) in a 5-level scale (female/somehow female/cannot guess/somehow male/male). In order to achieve an s-shape psychometric curve, we selected the faces that best fit the 0%, 20%, 50%, 80% and 100% male.