Smiling has been denigrated in lots of cultures but that doesn’t mean it isn’t a natural reflex. What it means is that the stuffy Roman aristocracy who left all the written records behind considered it undignified and beneath them - a sign of being common. Other cultures think smiling makes you look like an idiot. But even these cultures have an opinion about smiling so it seems to be present even though the culture works against it. This in fact suggests it is innately wired and not culturally learned.