A description of the images above made by Harvard psychologist Richard Russell, who won third prize in the 5th Annual Illusion of the Year Contest for them:

In the Illusion of Sex, two faces are perceived as male and female. However, both faces are actually versions of the same androgynous face. One face was created by increasing the contrast of the androgynous face, while the other face was 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 demonstrates that contrast is an important cue for perceiving the sex of a face, with greater contrast appearing feminine, and lesser contrast appearing masculine.

I found the following explanation much more useful and interesting though:

What you’re looking at isn’t an optical illusion, but is a play on the basic expected traits of men and women’s faces. The flusher lips of the left pic coincide with our expectations for women’s faces, as does the fairer skin. And it’s not just the illusion of lipstick; even without lipstick, we expect women’s lips to be more red than men’s. The difference in skin tone also brings to mind a recent a study suggesting that, on the whole, men’s faces are more red complected, while women’s are more green. Thus, even in the B&W photo, we infer that the darker complected face has the deeper reddish tone of masculinity; the lighter, the paler, greenish tone of femininity.

Obviously there’s much that’s debatable in that, especially whether those “expected traits” are universal or culturally-determined, but in the meantime I can’t deny that contrast is an important cue for determining the sex of a face, and that this provides more evidence for Korean women’s mania for lightening their skins being influenced by much more than merely wanting to emulate the wealth and sophistication represented by Caucasians.

Update) There is an 11-page PDF about these images available here, and you can find out more about Richard Russel and his research interests here.

(Thanks very much to reader Nicolas for passing this on)