How does a baby express being upset? They can smile on purpose at 2 months: https://www.uofmhealth.org/health-library/ue5463. So the lack of a smile is a detectable sign. After just a few weeks, they have a range of reactions short of crying. They change their facial expressions based on what they're feeling. They make grunting noises to express minor discomfort. You can even tell when they're bored.
> It's not an "ad hominem attack" to point out someone's apparent lack of experience with the subject of their comment.
It is if, as the other comment did, you just claim I don't know what I'm talking about without providing evidence of that.
But congratulations to you on using sources and arguments. I still think you're projecting. In my experience babies have no trouble recognizing human faces with masks on because they are drawn to eyes. In fact, they even recognize and will respond to a smile behind a mask, again because the muscles around the eyes contract. Probably masks are harder for slightly older children who are learning language, because they can't associate mouth movements with sounds.
On the merits--human babies are hard-wired to recognize faces from birth: https://news.stanford.edu/news/2012/december/infants-process....
How does a baby express being upset? They can smile on purpose at 2 months: https://www.uofmhealth.org/health-library/ue5463. So the lack of a smile is a detectable sign. After just a few weeks, they have a range of reactions short of crying. They change their facial expressions based on what they're feeling. They make grunting noises to express minor discomfort. You can even tell when they're bored.