Behind the numbers: Why do women earn less than men?

President Obama and his Democratic allies are hoping to win women voters by a large margin -- and part of that strategy involves making equal pay a big issue.

"If you do the same work as a man, you ought to be paid the same wage as a man," the president said at a reception honoring women from around the world in March of last year.

Research from the Bureau of Labor Statistics indeed suggests women on average make only 77 cents for every dollar earned by men.

Those numbers, however, could represent an apples-to-oranges comparison. The stats compare the pay of all women as a group against the pay of all men as a group, without considering lots of differences. Some analysts are now producing research that suggests the pay difference might be tied more to the jobs men and women each choose than their gender.

"The cardiac surgeon and the person who is checking out at a supermarket -- they are being compared as if they are equals," said Warren Farrell, author of the book "Why  Men Earn More" and a former member of the Board of the National Organization for Women in New York.

Pam Villareal, of the National Center for Policy Analysis, points to another difference -- the number of hours worked.

"So you might have a woman that works 35 hours, you might have a man doing the same job that works 50 hours, or he travels more in that job," she said.

Women are also less likely than men to work 40 hours, which would make a dent in women's earnings. "People who work 44 hours per week," Farrell said, "make 50 percent more than people who work 34 hours a week."

The reason for these and other differences is simple, they say. "Women are deciding not just what's great for their career, they're deciding what's great for their kids," said Kay Hymowitz of the Manhattan Institute.

That's not to say men never choose careers based on what's best for their kids, or that women always put kids before career.

But for women, there is a studied impact on their salary when they do choose to leave the workforce temporarily to have children and care for them.

Hymowitz said "there's no question that there is what some scholars call a 'motherhood penalty.'"

"It can be very difficult to get back in and earn the same kind of pay that you were earning before when you left," Villareal said.

Women are also less likely to take dangerous-but-better-paying jobs such as working in an oil field, coal mine or steel mill.

Beyond all that, there is one stat that could cast doubt on claims that working women are being discriminated against in the workforce.

"Young women in their 20s who don't have children, who are not married, are actually earning more than comparable young men," Hymowitz said.

Farrell said they're actually earning 117 percent more -- meaning women in that group earn 17 cents more on every dollar a man of the same age makes.