Patrick has often suggested replying to that question with some variation of "As a matter of courtesy to my former employers I prefer to not disclose the salary they paid me. I will, of course, extend you the same courtesy." and then back to establishing your value.

I have no gauge on how well that would work; it's been aeons since I've done a job interview. It's true that your previous salary is none of their business, though.

"Why is my previous salary relevant to what I offer you today?" Or something along those lines..

"Because you haven't improved much since yesterday so your salary should stay more or less the same" - they may answer :-)

"That salary was what I was worth when I started working for them. I've learned things since then. If you don't believe I'll be more valuable to you than to them, why are you hiring me?"

which is a non-answer.

your salary often has no bearing on what you're capable of.

If I'm working for a company that only knows how to extract $80k of value from my labor, and then only pays me $65k, that says as much about their ability to manage their business and resources as it does about my innate ability. My exact same skills might bring $250k of value to a different organization.

Paying me based on what I earned in the past is ... wrong.

They have no way to ascertain whether I have improved or not. ;-)

Plus what I might be offering them may be very different from what I offered my previous employers.

Is there any harm in lying? Given that it's confidential information that they can't really verify, and that they'll either pay you what you ask and be happy with you/fire you for not being worth it. Apart from moral concerns, what's the problem?

I've heard that it's not that uncommon to ask you to back your claim up with payslips.

True. It flags companies you shouldn't work for. They're asking for proof of your answer to their mean-spirited question. If that thinking has infected the whole company you'll be harmed in more ways later.

Lie, ethical when someone's asking an unfair question. You go into the interview knowing the appropriate value, which is generally what you want to make at the new job, assuming it's not at the extreme high end of the range and you have the skills to back it up.