Most persons see the face on the left as female and the face on the right as male. If this was your answer, then you are incorrect. If you thought the face on the left was male, then you are still incorrect. The truth is, there is no explicitly male or female face in the illustration. Both pictures are actually images of the same androgynous (possessing both male and female characteristics) face. The face on the left (usually perceived as female) has increased contrast in comparison to the face on the right (usually perceived as male).

This illusion demonstrates that contrast is an essential visual cue in the way humans process the sex of a face, with higher contrast being viewed as more effeminate. Women exploit and exaggerate this visual cue everyday through the use of makeup and other cosmetics in order to appear more feminine, delicate and beautiful.

Reference

Russell, R. (2009) A sex difference in facial pigmentation and its exaggeration by cosmetics. Perception, (38)1211-1219.