Massachusetts Gov. Charlie Baker, a Republican, signed the nation’s strongest equal pay law this week. It’s the strongest partly because it does something no other state has done before: It forbids companies from asking prospective employees how much they made at their last job before making an offer.

That’s a big deal when it comes to ensuring equal pay for women, advocates say.

Women make less than men even right out of college, and the wage gap only increases as they get older. One reason the wage gap widens with age is because women still take on more child-rearing and household duties than men.

But it’s also basic math: If you start off underpaid, and every new job pays you based in part on what your last job paid you, you'll pretty much be screwed until retirement — and you'll have less saved up for that, too.

What else does the new law do?

It bans pay secrecy

As in a number of other states, bosses in Massachusetts now can’t forbid their employees from discussing their salaries. The law also makes it easier for workers to sue if their boss unjustly fires or retaliates against them for discussing their pay.

Some employers either explicitly or implicitly ban the discussion of salaries, because they're afraid of stoking resentments or revealing inequities. And advocates say this lack of pay transparency contributes to the wage gap. Unconscious bias means women are frequently offered lower salaries for various reasons, and sometimes employers have no idea that they're consistently paying women less than men until they conduct pay audits.

If employees can discuss their salaries openly with each other, though, it can help draw attention to both intentional and unintentional pay inequity.

It requires equal pay for “substantially similar” work

The standard line you hear from politicians like Hillary Clinton is that we should have “equal pay for equal work.”

But Massachusetts’s law doesn’t just require equal pay for men and women who literally do the same job; it also requires equal pay for workers of different genders whose job requires “substantially similar skill, effort and responsibility and is performed under similar working conditions.”

Job title or description isn’t enough to determine whether a position is comparable. Other factors like seniority can also be included, as long as a woman’s lack of seniority isn’t related to pregnancy or family leave.

This is part of a big push from advocates to pass equal pay laws at the state level

A national equal pay bill, the Paycheck Fairness Act, is going nowhere in Congress — like most progressive initiatives these days. That's why advocates are turning to the states to try to build momentum for national action, and to help the workers they can in the meantime.

This movement has had some big successes, even in states with Republican governors like Maryland and Massachusetts. Several other states — notably Illinois, Minnesota, North Dakota, Tennessee, and Vermont — also have very strong equal pay laws.

Of the states that have passed strong equal pay laws recently, California and Maryland are the strongest behind Massachusetts. And while campaigners in both of those states tried and failed to ban asking about salary history, Massachusetts is the first state to succeed.

That doesn’t mean it will be the last, though. Advocates say they will keep pushing for similar provisions even in states where they’ve won recent victories, and Massachusetts will give them an example to point to.

Equal pay laws are also just one part of a multi-pronged women's agenda that advocates are pushing at the state level. This agenda also tends to include things like paid family leave, paid sick days, flexible scheduling, and affordable child care.

These proposals often come in packages, because advocates don't want to split them up and dilute their political power.

“It's easy to get caught up in issue policy silos,” Vivien Labaton, co-founder of the advocacy campaign Make It Work, told Vox in May. “But the issues people fall asleep worrying about aren't paid family leave over here, affordable child care over there. It really is an interconnected set of problems that people deal with on a day-to-day basis.”

The gender wage gap isn't a myth — it's just complicated

On average, women working full time in Massachusetts earn 82 cents for every dollar a man earns. The national figure is 79 cents to the dollar for women overall, but it’s a lot worse for women of color: 60 cents for African-American women, 59 cents for Native American women, and 55 cents for Latinas.

Critics of this figure say it's either an exaggeration or an outright lie. They say the gap basically disappears once you control for demographic information and the fact that women tend to choose lower-paying and more flexible fields, either because of child care responsibilities or personal preference for fields that happen to pay less.

But women don’t just “happen” to choose lower-paying fields; woman-dominated occupations are systematically devalued compared with those dominated by men, even for jobs that require similar education or skills. And when more women enter a previously male-dominated field, the overall pay in that field drops — for everyone, not just women.

It's true that the gap gets smaller depending on what you control for, but the gap never disappears entirely. Research from the American Association of University Women has found that even during the first year out of college, for people with similar educations, there is a gender wage gap:

Even controlling for demographic attributes that tend to affect earnings, AAUW found a 7 percent gap. In other words, a third of the gender wage gap couldn't be explained by other factors, and likely had something to do with bias or discrimination.

And even if we accept the explanations for other parts of the wage gap — that women sometimes work in lower-paying fields or often shoulder most of the child care responsibilities — we still can't ignore the role of gender bias in those realities. For more on that, check out my colleague Sarah Kliff’s stick figure explanation of the gender wage gap.