It depends,
I can't lie to make someone else feel good for example i cant say that someones hair or outfit looks good when it doesn't because either i will speak the truth before i can think about it or if i try to think about it my true feelings are written all over my face.

As a matter of principle I never lie in an attempt to achieve something over someone else, so a case of will not rather than can not

the closest to lying i get are half truths and lies of omission, but that's only when the topic of conversation is ME and the whole truth is too hard to say out loud.
So not really lying, just not ready for the truth

I've just noticed what I have as my status,says it all really and it's certainly what i'm going to try to do!!