NEW YORK (CNNMoney.com) - Men work more than women ... on the job anyway ... at least in terms of overall hours.

That's just one reason why when you make a general comparison of men's and women's earnings in most fields, men usually come out ahead, according to Warren Farrell, the San Diego-based author of "Why Men Earn More: The Startling Truth Behind the Pay Gap – and What Women Can Do About It."

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"People who do best in a field (financially) just plain put in more hours," said Farrell, a former board member of the New York City chapter of the National Organization for Women (NOW).

But hours alone don't fully account for the gap in women's and men's earnings. Farrell identifies 25 work-life decisions that men and women make in the course of their careers that have a direct bearing on their earning potential.

Farrell found, for instance, that men are more likely to opt for doing that which can lead to a higher paycheck, including:

• Relocate or travel extensively for work
• Take on more hazardous assignments
• Work in the hard sciences
• Take jobs requiring greater financial risk
• Work in unpleasant environments (e.g. prisons or coal mines)

Women, by contrast, are more likely to seek "careers that are more fulfilling, flexible and safe," Farrell writes. But the tradeoff is that "the pay can be lower because more people compete to be fulfilled, causing the supply to exceed the demand."

Even among highly paid women -- those who make over $100,000 -- Farrell found they are more likely than men at the same pay level to forfeit some pay in exchange for more free time. (And by "free time," I just mean free from the office. But often women reduce their work hours so they can take care of their families.)

While men may end up with more pay in many fields, Farrell believes women can end up with a better life on balance.

And in some fields, they have a shot at having a more balanced life and making more than their male counterparts.

He found more than 80 such occupations for regular working women. That is, not supermodels, who not surprisingly can earn far more than male models thanks to a seemingly insatiable demand for tall, thin, airbrushed versions of feminine beauty.

In 39 of the occupations Farrell found, women's median earnings exceeded men's earnings by at least 5 percent and in some cases by as much as 43 percent. (See the list here.)

Why do some fields pay women a premium? "No one really knows perfectly the answer," Farrell told me.

One factor may be scarcity. In fields like engineering, a company may get one woman and seven men applying for a job, Farrell said. If the company wants to hire the woman, they may have to pay a premium to get her.

That's because she may have more competing offers than her male counterparts. The reason: not only is she a top performer who can boost a company's profitability but employing her helps a company improve its equal-opportunity standing, which in turn can help it secure government contracts.

Also, where women can combine technical expertise with people skills – such as those required in sales and other arenas where customers may prefer dealing with a woman – that's likely to contribute to a premium in pay.

"She gives people the best of both worlds," Farrell said.

Another factor also may be an increase in the number of career programs designed to advance women, he noted.

It's not that Farrell doesn't think pay discrimination exists. It does, he said, but it's not always against women. There's plenty of it against men, too.

He points to careers that have limited opportunities for men – e.g., dental hygienist or elementary school teacher because people prefer a woman in those roles. That's not that much different from law firm clients who years ago may have preferred to deal with a male lawyer, he notes.

I don't usually use this column to recommend books, but "Why Men Earn More" will provide much food for thought, no matter where you stand in the pay-gap debate.

And for anyone inclined to assume men have it easier at work, this book -- complete with far more textured, subtle arguments than a column can ever convey -- will make you think twice.

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Jeanne Sahadi writes about personal finance for CNNMoney.com. For comments on this column or suggestions for future ones, please e-mail her at everydaymoney@cnnmoney.com.