As explained in this article by Stacy Cowley, Massachusetts has become the first state to bar employers from asking job applicants about what they earned at their prior jobs. This excerpt explains why:
By barring companies from asking prospective employees how much they earned at their last jobs, Massachusetts will ensure that the historically lower wages and salaries assigned to women and minorities do not follow them for their entire careers. Companies tend to set salaries for new hires using their previous pay as a base line.
And the law
will require equal pay not just for workers whose jobs are alike, but also for those whose work is of “comparable character” or who work in “comparable operations.” Workers with more seniority will still be permitted to earn higher pay, but the law effectively broadens the definition of what is equal work.
The bill had bi-partisan support and was signed by the commonwealth's republican governor. Worth reading in full.