While I disagree that "The boy version of the emote looks like he's losing his sanity, and is about to throw a chair across the room," I do also feel that there should be consistency with the male and female emotes. The boys and the girls should both have the same smile.
In almost all AC games (Can't speak for GC), male humans have wider mouths than females. Not only that, but it isn't entirely uncommon for certain cartoon styles to give females smaller mouths/males larger mouths, so it's possible it's just the artist's rendition/freedom of, well... mouths (And now I am curious who made the emotes).
So.. there isn't anything to "fix", really. It's intentional.
I think this idea is worth discussion, but as someone pointed out before these emotes have been the same for some time and some users might have become attached to their appearance. I would also like to point out that the eyes/eyebrows? are also different for the boy and girl smiling emote and only reducing the size of the smile might not end up with an emote that looks calm or stress free like the girl.
I don't know all that goes into changing emotes, but I think that if this idea gains some support it is something that can possibly be discussed to change.
The 's smile emote was actually changed a few years ago to what you're seeing now. It used to be just like the 's smile. Titandwedebil did this change and also created some of the newer emotes. Not sure if he still has the old smile one or not. This could probably be voted via poll.
I personally don't see it as a huge issue, to be honest. Emotes are meant to help express how someone feels or show the tone of someone's message. So I don't think its appearance is that much of a problem.
It would be a bit more professional if the smiles were the same on the male and female emotes, but it doesn't cause any problems as far as I can see. I didn't even notice the difference until it was brought up.
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