"It's just gas."

There exists the idea that newborn babies don't really smile to show that they're happy, it's just their face happens to land in that position sometimes.  A common explanation is "It's just gas."

I can't tell you if this is true or not.  There are people on the internet who confirm this statement and saying that it's a common misconception that babies really smile, and there are people on the internet saying it's a common misconception that babies don't smile and they totally really do.

But the pervasiveness of this idea that babies don't smile means that once upon a time someone discovered it or thought they discovered it, and then they - and many other people - perpetuated the idea.

Why would someone do this??

If you, as the adult, think the baby has smiled at you, you're happier.  You feel "Awww, she likes me!" and that brightens your day.  Your happiness may make the baby feel happier, safer, or more relaxed, or it may be neutral, depending on how well the baby can read your moods and how much they affect her moods.

If you think the baby likes you in general, you're more likely to want to engage with her, and more often, so you can see her smile again.  Engaging with the baby is good for her social development and her language development, and will probably help her get to know you better and build trust with a loving adult.  Plus, if you genuinely feel that the baby likes you, you're more likely to respond sympathetically to her.  When she's crying, you're more likely to go "Awww, poor baby!" rather than "Shut up, you ungrateful little brat!"  This will make her feel safe and secure and loved, which is good for her long-term social and psychological development.

In short, thinking that the baby has smiled at you makes life a little happier for everyone and, even if it's not true, there's no downside. Conversely, if you think any sign of affection from the baby is just instinct or fluke or gas, the best possible outcome is neutral, and the worst is neglect.

So why would someone do science with the goal of proving that the baby doesn't really like you?  Or, if they discovered this by accident, why would people work so hard to perpetuate it?  Even if the truth is that the baby doesn't actually like you because she's too young to like you, there's nothing lost and a certain amount gained by being deluded into thinking she likes you.