The article notes that the babies are with their mothers most of the time, so they're probably exposed to plenty of language even if it not specifically aimed at them.
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?