Of course, employers who inquire into salary history generally claim that they need to know what you've earned in the past because it helps them figure out how much you should be earning now, or so that they can screen out candidates who are earning far more than the position pays and presumably won't want to take a pay cut. But neither of these reasons holds water. First, companies should be able to determine a candidate's value for themselves; they don't need to look to their competitors to tell them a candidate's worth. And second, if they're concerned that you'll be unhappy with the salary they're offering, they can solve that by posting their range up-front or ask you about your salary expectations rather than salary history. Demands to know your past salary are designed to give employers the upper hand in salary negotiations.