Why does a woman earn less than a man doing the same or similar work? Ask their employer, and there will always be a good reason. But are those reasons really good enough?

The internet is buzzing this week with Catt Sandler's recent departure as an anchor for E! She explained on her website that she quit after learning that her co-host on E! News, Jason Kennedy, was earning nearly twice as much as she was even though they both started at the network the same year. (According to an independent report, her salary is likely around $600,000 a year, and his is around $1 million to $1.2 million a year.) 

With her contract up for renewal, her team attempted several times to negotiate a salary closer to Kennedy's, but E! refused each time, she wrote. So she chose not to renew. Although she loved her job, she explained, she couldn't in good conscience stay on at the network and condone the unequal pay. "How can we make it better for the next generation of girls if we do not stand for what is fair and just today?"

E! has so far only responded with a written statement to some media outlets that said, "E! compensates employees fairly and appropriately based on their roles, regardless of gender," and went on to wish Sadler well. But others have defended the network. One source "close to" E! told Us Weekly that Kennedy made more because he was the lead anchor on E! News while Sadler was the second anchor.

Apparently, when Giuliana Rancic held the lead anchor position and Kennedy was second anchor, she made about three times what he did. "I believe in female empowerment and gender equality, but it's heartbreaking that a network is being deemed sexist when that is simply not true," Kennedy's wife posted to Instagram.

Is the fact that Kennedy is lead anchor and Sandler was second anchor a good enough explanation for they pay disparity? I don't know, and honestly, I don't care. The gender pay gap is not about Catt Sadler and Jason Kennedy, it's about the more than 70 million women in the U.S. work force, who collectively earn about 80 cents for every dollar earned by a man. 

As E!'s defenders demonstrate, when it comes to an individual woman making dramatically less than a man doing the same or a similar job, you can pretty much always find a non-gender explanation. But when it comes to the fact that women in the U.S. in general earn less than men, those explanations start looking pretty thin. Here are a few of them:

1. Women earn less because they choose lower-paying professions.

No. There's plenty of evidence that women in high-paying professions (lawyers, for example) don't earn as much as their male colleagues. And that many low-paying professions are low-paying because they're dominated by women, rather than the other way around.

2. Women don't work as hard as men.

That may be true. In one study, men were much likelier than women to report working more than 50 hours a week. But this accounts for less than 10 percent of the pay gap, leaving the other 90 percent to be explained.

3. Women don't ask for more money often enough.

This is such a widespread myth that I believed it about myself when I heard about male counterparts earning more than I did for the same work, even though I had seniority. But in fact, a large study in Australia showed that women ask for raises just as often as men do--and are turned down more often than men. (The study was done in Australia because it's the only nation that keeps official records of people asking for raises.) 

4. Men earn more because they have more education.

It's too bad this reason isn't true. If it were, the gender pay gap would disappear in the relatively near future since women make up more than half of U.S. college students. But a Georgetown University study showed men with an incomplete college education earn more on average than women with a B.A. A woman needs a Ph.D. to reach pay equity with a man who has a B.A.

5. Men earn more because they've spent more time in the work force.

If that were true, the gender pay gap would decrease for women who've spent many years on the job, but instead it increases with every year women spend working. Incidentally, this also disproves another common excuse--that employers give women less responsibility (and correspondingly lower pay) out of fear they'll leave their jobs to raise children. If that's true, why do women's salaries continue to fall further and further behind men's after their childbearing years are behind them?

Here's the real reason.

There's only one reason employers pay women less than they pay men and it's a very simple one--because they can. Think about it. Do you ever pay more for a product or service when you can get the same item at a lower price? Why would you? Employers are accustomed to paying women less than their male colleagues, and women--who have never had pay equality in this country or anywhere else--accept those lower salaries because we mostly have no choice. 

There's a basic math problem standing in the way of gender pay equality. Women earn about 80 percent of what men do, and make up 47 percent of the U.S. work force. Assuming they don't cut pay for male employees, closing the gender pay gap would mean U.S. employers in total would need to increase their payroll expenses by 9.4 percent, not counting increases in related non-salary expenses such as taxes. Could your company afford to increase its total salaries by more than 9.4 percent?

Most employers would say no. But they would really mean that they very much don't want to. Faced with the choice between spending more on their work force or shutting down, the vast majority of companies would find a way to pay up. In fact many have done just that as the current tight labor market propelled salaries for some skilled positions to new heights.

We may not reach gender pay equality until women in large numbers do what Sadler did and refuse to work for employers who pay them less than their male colleagues. Most working women don't have the luxury of making that choice. But with women earning more degrees than men and making up of a significant portion of that hard-to-hire skilled work force, there's never been a better time for at least some women to make that stand. If they do, they could wind up changing the work world for all of us.