could care less // couldn't care less

[Daruma]

Merriam-Webster's Learner's Dictionary
could/couldn't care less
informal ◊If you could care less (US) or couldn't care less, you are not at all concerned about or interested in something.
▪ I could care less what happens. [=I don't care what happens]
▪ He says he couldn't care less if he gets fired.
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1. I could care less what happens.
2. I couldn't care less what happens.
3. He says he could care less if he gets fired.
4. He says he couldn't care less if he gets fired.


In American English, do they prefer #1 to #2?
In American English, do they prefer #3 to #4?

[FarihaS]

Hello Daruma,

In AmE, "couldn't care less" is usually used. But I believe there is not much difference. You can use either!

Hope this helps!

[konungursvia]

Could care less is sort of a quick abbreviation for "couldn't care less."

[Raymott]

In American English, do they prefer #1 to #2?
In American English, do they prefer #3 to #4?

Look here:
http://alt-usage-english.org/excerpts/fxcouldc.html