Research by neuroscientist Richard Russell of the Gettysburgh Perception Lab suggests that sexual dimorphic differences between male and female faces, especially under circumstances of minimal sexual dimorphism, is all about contrast. Males have darker skin except for eyes and lips. Comparisons of manipulated androgynous faces demonstrate that greater ratio of skin:eyes/lips contrast suggests female, while lower ratio of contrasts suggests male. The effect of cosmetics (e.g., mascara and eyeliners that darken eyes, lipstick that distinguishes lips) is to increase the contrast and thereby enhance the appearance of femininity, whereas lessening cosmetics enhances the appearance of masculinity.
NEW! Evolutionary Tidbit of the Moment
In seemingly unrelated languages from every corner of the globe, the word corresponding to "mother" contains a sound like /ma/, as in "amma," "mama," or "ima." Father words tend to have the /pa/ or /ba/ sound, like "appa," "abba," "baba," or "papa." A discarded hypothesis held that the words for "mother" and "father" had remained largely unchanged from a proto-language from which all modern languages evolved.
The currently favored explanation is that these are the first sounds infants are able to make, with /m/ being slightly easier (and thus developing sooner) than /p/ or /b/, explaining why the primary caretaker (usually the mother) tends to be referred to by words which sound like "mama" in languages all the world over.