NEW YORK (MarketWatch) — We hear it over and over again: the myth of the gender wage “gap.”

Here’s President Obama, speaking on June 4: “And we’ve made progress. But we’ve got a lot more to do. Women still earn just 77 cents for every dollar a man earns.”

An Obama campaign TV ad, entitled “First Law,” which began airing June 21, showed the same 77 cent figure.

Just one problem — it isn’t true. Here are three myths about the wage “gap.”

Myth 1: Women get less pay for equal work. The spurious assertion that women are paid 77 cents for a man’s wage dollar comes from comparing the earnings of all full-time men with those of all full-time women.

The comparison is bogus, for two reasons. First, it lumps together men and women who work different numbers of hours — any hours above 35 hours per week. On average, full-time women work fewer hours than full-time men, often because they prefer it.

When comparisons are made between men and women who work 40 hours per week, women make 87% of men’s earnings, according to the Labor Department. For men and women who work 30 to 34 hours a week, women make more, 109% of men’s earnings.

Second, the gap claim averages for each gender earnings from many and disparate vocations. For example, it averages women who work as social workers with men who work as investment bankers; female elementary school teachers with male engineers; and male loggers with female administrative assistants.

For their own reasons, many women enter so-called “helping professions,” such as nursing, teaching, elder care, health services, nutrition, social work. These occupations pay less than do some more dangerous and physically-demanding lines of work that attract more men — engineering, mining, operating construction machinery.

Legitimate comparisons look at men and women with the same job tenure in the same position at the same firm. If there’s a big difference under those circumstances, there may be discrimination, giving women grounds to sue. Federal law forbids discrimination, and permits such suits.

When economists compare men and women in the same job with the same experience, the analysts find that they earn about the same. Studies by former Congressional Budget Office director June O’Neill, University of Chicago economics professor Marianne Bertrand, and the research firm Consad all found that women are paid practically the same as men.

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President Obama says he’s in favor of equal pay. Does he practice what he preaches? Not according to my calculations from 2012 pay data published by the White House. I found that women staffers there were paid 91 cents on a man’s dollar — if one calculates the figure, incorrectly, based on simple averages by gender. This is presumably because female staffers in these offices were not as senior as male staffers, or they held different types of positions, just as in the workforce as a whole.

Myth 2: Women are discouraged from enrolling in higher-paying fields — science, technology, engineering, math. Not true. No one prevents women from taking the curricula they prefer to get ahead.

However, fewer women choose to major in engineering, chemistry, and physics. More choose to take English literature, communications, and gender studies. Graduates in these fields are usually paid less than in the sciences.

Data for degrees awarded show that women are scoring ahead of men. According to the National Center for Education Statistics’ Digest of Education Statistics, women were projected to get 58% of masters and bachelor’s degrees, and over half of PhD degrees for the 2011-2012 academic year.

Data on the courses of undergraduate study which women choose reveal their vocational preferences. In 2010, the top five woman-heavy majors were family and consumer sciences/human sciences (88% female); library science (87%); health professions and related programs (85%); public administration and social service professions (82%); and education (80%).

The top four man-heavy majors are more highly paid but draw relatively few women. They were military technologies and applied sciences (4% female); transportation and materials moving (11%); engineering and engineering technologies (17%); computer and information sciences and support services (18%); and economics (30%).

Leah Loversky, a senior at Pomona College in California majoring in economics, told me that most economics majors on her campus are men. Last semester, in a 21-student class on game theory, she was one of two women. (She got an A minus.) “No one tried to discourage me from taking the course,” she said. “In fact, my fellow economics majors all encouraged me to take it.”

Women who prepare for science and engineering are well rewarded in a job market that traditionally has been male-dominated. One 2010 study found that while women represented 11% and 12% of university tenure-track applicants in electrical engineering and physics, they received 32% and 20% of job offers. They were more likely than male applicants to get hired when they applied. This shows that in the sciences, employers seek to remedy the traditional gender imbalance by seeking out bright women, who benefit from affirmative action. Read more about the study.

Myth 3: A discriminatory “glass ceiling” restricts women to lower-paying jobs and careers and keeps them out of senior management and the corner office. Many women, even those with excellent academic credentials, prefer to work part-time in order to combine work and family. Family-friendly jobs with flexible hours pay less than jobs with longer, inflexible hours. (Some feminists contend that this is unjust, but that is a separate issue.)

It’s not the “glass ceiling” that keeps women out of the corner office, it’s a choice of how much time and effort to put into one’s career. Many in the millennium generation (born after 1980) call it “work-life balance.” For men and women, to make it to the corporate top requires countless hours of work and travel and too little time for family. That means missed birthdays, football and field hockey games, and school productions. Women seem to mind missing these events more than men.

Consider women at Yale Law School. In 2012, as it has done in many other years, Yale Law Women, an organization of female law students at Yale Law School, made a list of “Top Ten Law Firms,” in categories particularly noted for family friendliness. Read more about the Yale Law Women list.

They picked firms that offered part-time and flex-time work, as well as generous parental leave. “One of the goals of the Top Ten list is to generate discussion about family- friendly policies at top law firms,” Yale Law Women wrote on its website.

These are women who have the credentials to aim for the executive suite at major corporations, but some are planning for part-time and flex-time. There’s no problem with those choices, but these same women shouldn’t cry discrimination when they don’t make it to the top.

Myths and realities — women and men grow up with them. Some myths teach us moral and ethical truths, and we are the richer for them. But when myths try to teach us something demonstrably false — such as women earning less than men for the same work — we are all the poorer. It is time to discard false myths about women.

Diana Furchtgott-Roth is an economist who served in the administrations of Presidents Reagan, George H.W. Bush and George W. Bush, most recently as chief economist of the Department of Labor from 2003 to 2005. She is a senior fellow at the Manhattan Institute and a contributing writer for various publications, includingRealClearMarkets.com , the Washington Examiner and Tax Notes. She is co-author of Women’s Figures: An illustrated Guide to the Economic Progress of Women in America.