Check this out Scientific American’s best illusions contest.
This photo is fun. Which image appears to be female?
It’s the same photo, with the only change being the contrast!
“The Illusion of Sex demonstrates that contrast is an important cue for perceiving the sex of a face, with greater contrast appearing feminine, and lesser contrast appearing masculine.”
In this case, the full colored lips of the left hand photo makes it appear female, a facial recognition features that even babies use to distinguish gender.
Hence, the widespread use of lipstick!
Psychological studies suggests that this is a primal response, hardcoded into our DNA. The more you can make your lips look like an enflamed vaginal opening, the more female you will appear . . .