Should it be illegal to ask candidates their current salary?

The state legislators of Massachusetts think so - they have just passed a law prohibiting employers from asking this question (although the candidate can still volunteer the information) and requiring the hirer to disclose the salary being offered upfront.   This is part of a package of measures aimed at reducing the gender pay gap. 

There is some evidence to show that negotiable salaries tend to result in women earning less, so requiring the salary (or salary range) to be disclosed upfront may be useful in this respect, although it could also be very helpful for competitors looking to poach staff by enabling them to deduce the company's salary structures.  Likewise, limiting the ability of employers to base someone's salary offer on what they are currently earning may help to reduce baked-in disadvantage - but, at the same time, creates an imbalance of information between a company which is hiring and its competitors.

It would be interesting to see whether the UK government, which has made some very public promises about reducing the gender pay gap, will be tempted to follow suit, or whether it would see this as too much of an interference in salary negotiations.