see that cute chick smiling? Watch the video here:
here's a quick ethology lesson. For females, it's more attractive to males if she smiles a lot. For males, it's more attractive to females if he doesn't smile that much.
this is why, when you see photos of women, they are all smiling and kissing. While, males, you don't see smiles. Go head, check all your friend's photos on social networks.
same thing applies to frequency and degree of facial expressions. Females, you want to be very expressive. Males, cool dude look is good for you. You can see this in every day life, or in TV, movie. This video is also a good example. See that chick, smiling and expressiveness of her face.
of you academic types, there are psych studies on this too. Just google.
(thx to Paul Snively)