Could care less or couldn’t care less?

Which is correct?

I’d like to use responsive design to make sure our site renders well on mobile devices, but our lead developer ______________________.
a. could care less
b. couldn’t care less

Answer: The correct answer is choice b.

In everyday usage, both phrases are used as a substitute for “does not care.”  In written English, however, the correct answer is choice b.

Reason:  The phrase “I could care less” implies that you do care, although it’s possible for you to care to a lesser degree.

The phrase “I could not care less,” on the other hand, means  it is impossible for you to care any less than you already do, which in turn means that you do not care at all.

Follow Ivy Wigmore on Twitter @tao_of_grammar