No smile in passport photo either:
Passport Photo Requirements
...
With a neutral facial expression and both eyes open
...
http://travel.state.gov/passport/pptphotoreq/pptphotoreq_5333.html
I must have missed that line in the requirements. I’m grinning from ear to ear in my passport photo.
It may have to with facial recognition software but you can always change what your teeth look like. Maybe if an LEO looks at your photo and your smiling they’ll be more lenient with you and of course we can’t have that.