Oh sure, but they're presumably not using sing-song baby language, or perhaps singing songs all that much.
The sing-song thing is cultural but powerful. It’s hard (for me) to not default to it with my one year old.
Edit: LOL I clearly commented before reading the article.
My actual question is … how to develop good humor and playfulness in kids? Some people grow up with somewhat stunted senses of humor it seems. Is there something one can do? Research?