TL;DR: Babies laugh in an interactive or social context. When adults around them laugh, they laugh with them for example. They look at cues that others are laughing and are happy, so simply watching someone trip and crack their head on the floor won't be funny usually (it would probably register as dangerous and scary).
Of course the important thing that is not mentioned explicitly is that it makes adults, especially parents, happy and it rewards them, so laughter becomes a bonding experience. Parents like to hear their baby laugh, they make the baby laugh, they are happy and baby is happy. And as a result baby bonded with their caregiver.
I don't remember when my daughter made her first steps on her own, or the moment she said "daddy", but I remember vividly the context when she belly laughed for the first time. For whatever reason that was a very wonderful and memorable moment that I hope I'll never forget.
I vividly remember the first "joke" I shared with my infant son when he was about 1-1.5:
He was pointing to colours and my job was to say it out loud. At one point, I deliberately called black as "green" and he instinctively skipped to the next color but quickly did a double take and jumped back to the previous one, thinking he misheard it. I repeated "green" and he looked at me and started laughing (I was expressionless).
Mind, this was the first time I had done something like this in any context. And he got it! It was a joke.
Now, at about 5, he gets puns and sarcasm is hit'n'miss. As a side-note, the whole thing as to how his appreciation for jokes is evolving is interesting. But the most important thing for me as a parent is enjoying these moments of laughter.
Wow, impressive story! To myself, understanding that particular joke requires quite a lot of high level cognition. What an enjoyable moment that must have been. :) I wonder if sarcasm/irony detection is at least partially an inborn trait.
The baby didn't have to understand the joke, it could have just picked up on unintentional cues. This is something humans and many animals are able to do very early on.
I have a 10 month old and this feels right but I have zero data to back it up.
One thing I tried as an experiment turned into something good. When a baby falls, bumps their head, hits themselves in the face (remember: no motor control), their first impulse is to scream. Sort of.
I noticed that when ours falls, sometimes he looks at us before reacting almost as if he's waiting for a clue on how to react. I clap and cheer and instead of freaking out, he just smiles and goes back to what he was doing.
This only seems to work when surprise and not pain is the cause. If he bonks his head, it's still the end of the world. ;)
"Via his website he surveyed more than 1000 parents from around the world, asking them questions about when, where and why their babies laugh."
I hope this is just confusion through the lens of the BBC, but... It sounds like they had people self-report when they observed babies laughing, and found that babies laugh when being observed?
I'm not really sure I'm convinced by the results. I'm not a researcher on infant laugh but in my son's case I think it's mostly due to him trying to mimic the expression his parents have.
If you smile an infant smile if you make a sad face he'll be sad. At least that's my it from my experience
This explanation seems overly narrow. Some novel sensations are just funny to babies, like the sound of ripping paper [1], tickling their brain as it were. (Okay, I wouldn't be surprised if could devise a study to show that the sound of ripping paper alone doesn't make a baby laugh.)
I took it as discovering an unexpected, seemingly uncharacteristic quality of something important.
A baby that sees elders consistently and intently look at paper, fold it carefully, put it in important places, then all of a sudden sees one purposefully break it would give a jolt.
Either way, I love that video and have wondered what else might give a baby a similar experience.
Though my 7 month old isn't a baby anymore, it's pretty clear to most people that she knows that she has a great sense of humor. Not only does she understand what's funny and what's not (subtleties in expectations and anticipation), she makes up her own jokes, as well.
Of course the important thing that is not mentioned explicitly is that it makes adults, especially parents, happy and it rewards them, so laughter becomes a bonding experience. Parents like to hear their baby laugh, they make the baby laugh, they are happy and baby is happy. And as a result baby bonded with their caregiver.
I don't remember when my daughter made her first steps on her own, or the moment she said "daddy", but I remember vividly the context when she belly laughed for the first time. For whatever reason that was a very wonderful and memorable moment that I hope I'll never forget.