Motion information plays only a secondary role in sex identification of walking persons in frontal view

Observers have a success rate above chance in identifying the sex of walking persons on the basis of movies showing only point lights. It has been claimed that observers rely heavily on motion information for their judgment. Here, we studied, for the frontal plane, the added value of motion information over just form information. In the first experiment, we asked 209 observers to identify the sex of frontal-plane still images of point lights of six male and six female walkers. We used two types of point-light images: (1) cloud-like images, showing just point lights, and (2) skeleton-like images with point lights interconnected. On the basis of cloud-like still images, observers had a mean success rate of 63%; on the basis of skeleton-like still images, they had a higher mean success rate of 70% (p < 0.001). In the second experiment, we asked 273 observers to identify the sex of skeleton-like still images and skeleton-like movies of eight full walking strides. The overall success rate based on movies was 73%. Among the observers first presented with still images, the success rate based on still images was 68%, but for observers first presented with movies the success rate based on still images was 74%, not different from that based on movies (p > 0.05). Our interpretation was that motion information revealed what the point lights represented but had no additional value when this became clear. Hence, we concluded that motion information plays only a secondary role in sex identification of walking persons in the frontal plane.