I would like to do some more scientific exploration of this concept, how female do you need to make a face before we see it as such. Being able to say a face is gender neutral is nice, but if pushed which way to people fall for something with as few clues as a smiley.
I did find a few articles/papers talking about different aspects of this, but not right on point. The virtual facial feminism site covers the topic from a different angle while my look at the Simpsons below shows how this style uses more blatant cues to distinguish gender. Check out the Simpsonized casts to see how they push the art style to make likenesses of male and female characters. In the corner I simply took away the eyelashes and shortened the hair, is the character still female? Do we need extra cues like these to set the gender.