You know the one. They go up an octave or two, and use that sing-song voice specifically reserved for babies and puppies.

Do you suppose it's nature or nurture? Seems like every culture on the planet does this.

Men have it too. Yes, it's probably instinctual rather than learned seeing as, like you said, every culture on the planet does it. Especially since I know when I do it, I don't do it on purpose.

You also might have noticed that some people also use a fake/higher voice when they're on the phone with someone who isn't a good friend or relative.

I laugh at this all the time

This must be nature. I try to make a point NOT to do it, but my voice turns to jello with babies and other small, fuzzy animals.

Its been proven several times in nature that low frequency voices are meant for intimidation and vice versa. *cue death metal*