We have a 12 year-old with ADHD, and I suspect he's a pathological liar; we call him out on it all the time, but even then, when we ask him a question - even innocuous things like "have you had breakfast" if he's up before we are - his first instinct / default is to answer with what he thinks we want to hear. He knows he doesn't get away with it in most cases.

I suspect there's two factors at play; one that he sometimes gets away with it, where we can't be bothered to double check, and second that he may get a little 'thrill' from lying and getting away with it, moreso than the adverse reaction from being called out on it. Mind you, that said, negative attention is also attention, so subconsciously for him it's likely a win-win.

Anyway, it's still annoying. It's not (yet) serious things, just mainly some responsibility dodging behaviour.