two faces are perceived as male and female ..... though both faces are actually versions of the same androgynous (asexual) face!The left hand side face is created increasing the contrast, while the other i.e. right side face is created by decreasing the contrast. The face with more contrast is perceived as female, while the face with less contrast is perceived as male.
The illusion of sex | created by Richard Russell of Harvard University, USA
The Illusion of Sex Change demonstrates that contrast is an important cue for perceiving the sex of a face, with greater contrast appearing feminine, and lesser contrast appearing masculine.
BTW, this sex illusion won the third prize in 2009 optical illusion contest held by neuralcorrelate.com