Why is it that women never smile in their photos?

I never see girls smile in their photos. They usually take a photo of them alone, with a "gang pose" or some other dorky pose, by their car, with a "tough look" on their face. Do they do it to look "cool"? They always have a mean or dull look on them.

Guys are the opposite. They always smile in their filtered photos. They also take photos with large groups of friends and hold each other in their photos. They always look so happy.

Why is this?
Why is it that women never smile in their photos?
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