There's some studies out there which claim that beta monkeys/chimp will bare their teeth in submission to an alpha.

I've seen threads critiquing someone's appearance here, and the first thing usually said is "lose the toothy smile"

But why do modern guys do this so much? If you look at early photographs, nobody is smiling. Could it be all this smiling is the result of the dental and personal hygene industry trying to get us to buy into whatever they're selling?

Either way, I've been making an effort to never do a toothy smile again. I only smirk or do a straight face now.