When I’m doing an intense activity, I often scrunch up my face into a scrunched, toothy grin. I also do this during pain, as in the phrase “grin and bear it.”
I do this unconsciously. I can’t think of a good evolutionary or practical reason for it. So, why do we do it?
I don’t think anyone would mistake this ‘grin’ for a jovial one. So ‘grin’ is just a poverty of vocabulary - it’s a different facial expression. ‘Grimace’ is maybe clearer.
Most bared-teeth expressions in primates have to do with fear/anxiety and/or aggression.
So my guess - and it is a guess - I have no citation - is it’s a natural response to anxiety.
Oh - you asked for an evolutionary reason. Most all facial expressions are for communication. We’re a social species. It’s helpful for others to know what we’re experiencing. Again, that’s a reasoned guess. No citation.
It’s not a grin. It’s a grimace.
There’s still the issue of the cliche cited by OP: “Grin and bear it.” Why grin, if the situation needing bearing would justify a grimace?
I don’t have a take on that; at least, not one I’d care to put forth and defend (or even research). But that question, at least, remains.
I never understood the expression “grin and bear it” to relate to the actual, natural facial expression when faced with an unpleasant situation. I always understood it to be “put up with, toss on a smile and just deal with it.”
Fig. to endure something unpleasant in good humor. There is nothing you can do but grin and bear it. I hate having to work for rude people. I guess I have to grin and bear it.
Never mind. Raventhief beat me to it.
Interesting point. I think I made my own interpretation of the phrase that is different from the original intent.
Grinning (or smiling) opens the trachea by relieving pressure from the muscles of the neck and causing the head to tilt slightly backwards. Frowning causes the head to tilt forward and tense the muscles of the neck. I mean you can buck these effects, but that’s what I’ve read is the unconscious effect.
This is a well recognized phenomena in dance. Teachers tell students to smile to relieve stress, improve their balance, and breathing.
When I’m doing a difficult skating maneuver I always smile to do all of the above. It works.