If, for the same work, women make only 77 cents for every dollar a man makes, why don’t businesses hire only women? Wages are the biggest expense for most businesses. So, hiring only women would reduce costs by nearly a quarter – and that would go right to the bottom line. Don’t businesses want to be profitable? Or, are they just really bad at math?

Well, actually, it’s the feminists, celebrities and politicians spreading this wage gap myth who have the math problem.

Here’s why:

The 77-cents-on-the-dollar statistic is calculated by dividing the median earnings of all women working full-time by the median earnings of all men working full-time. In other words, if the average income of all men is, say, 40,000 dollars a year, and the average annual income of all women is, say, 30,800 dollars, that would mean that women earn 77 cents for every dollar a man earns. 30,800 divided by 40,000 equals .77.

But these calculations don’t reveal a gender wage injustice because it doesn’t take into account occupation, position, education or hours worked per week.

Even a study by the American Association of University Women, a feminist organization, shows that the actual wage gap shrinks to only 6.6 cents when you factor in different choices men and women make. And the key word here is “choice.” The small wage gap that does exist has nothing to do with paying women less, let alone with sexism; it has to do with differences in individual career choices that men and women make.

In 2009, the U.S. Department of Labor released a paper that examined more than 50 peer-reviewed studies and concluded that the oft-cited 23 percent wage gap “may be almost entirely the result of individual choices being made by both male and female workers.” Well, let’s look at some of those choices.

Georgetown University compiled a list of the five best-paying college majors, and the percentage of men or women majoring in those fields:

Number 1 best-paying major: Petroleum Engineering: 87% male Number 2: Pharmaceutical Sciences: 48% male 3: Mathematics and Computer Science: 67% male 4: Aerospace Engineering: 88% male 5: Chemical Engineering: 72% male

Notice that women out-represent men in only one of the five top-paying majors – by only a few percentage points.

Now consider the same study's list of the five worst paying college majors:

Number 1: Counseling and Psychology: 74% female Number 2: Early Childhood Education: 97% female 3: Theology and Religious Vocations: 66% male 4: Human Services and Community Organization: 81% female 5: Social Work: 88% female

Here, it’s the women who lead in all but one category. Even within the same profession, men and women make different career choices that impact how much money they make. Take nursing, where male nurses on the whole earn 18% more than female nurses. The reason? Male nurses gravitate to the best-paying nursing specialties, they work longer hours, and disproportionately find jobs in cities with the highest compensation.

Now, here’s how one expert on nursing compensation, Professor Linda Aiken of the University of Pennsylvania, sums up the data: “Career choices and educational differences explain most, if not all, the gender gap in nursing.”

The Department of Labor paper concluded that once these differences are accounted for across all professions, the unexplained wage gap is somewhere between 4.8 and 7 percent – almost identical to the 6.6 percentage gap found by the AAUW.

But why is there any gap at all? No one knows for sure, as both the AAUW and the Labor Department concede. There are so many variables that drive wages that no single study can cover them all. Few wage gap studies control for variables such as dangerous work environment; men are vastly overrepresented, for example, on oil rigs. And here’s another variable: men are more willing and able to work long hours without advance notice.

According to Harvard economist Claudia Goldin, even if two lawyers have the same education and same specialty and work the same number of hours – firms pay more to someone who is willing to always be “on call” and ready to be in the office when the firm needs them, as opposed to wanting a more regular schedule. This isn’t sexism, it’s just common sense.

With more realistic categories and definitions, whatever wage gap remains would certainly narrow to point of vanishing.

So it seems that business leaders aren’t bad at math simply because they don’t only hire women. Those who claim that for the same work women earn 77 cents on the dollar compared to men, on the other hand, are not merely bad at math – but at telling the truth.

I’m Christina Hoff Sommers of the American Enterprise Institute for Prager University.