"I couldn't care less" is the proper American form and the only form I heard growing up in the US. In recent decades, I have of course encountered the "could ca

"I couldn't care less" is the proper American form and the only form I heard growing up in the US. In recent decades, I have of course encountered the "could care less" form many times, but it's like "same difference" meaning "same thing": something that professional copy editors (which I used to be) will still probably repair unless it's being used to paint a picture of a character.