Ten years ago I gave birth to two babies: one girl, one boy. Their birthday is today, Equal Pay Day.

Right now they are neck-in-neck with their weekly allowance, but if they were fifteen years older, my daughter's college degree, MBA, technological training, organizing and management experience, and even her Rhodes Scholarship would all probably be worth less than those of her brother. Why? Because there is a stubborn pay gap between males and females that persists even among those with equal credentials. On average American women earn 77 cents for every dollar American men earn.

Unequal pay persists for many reasons. Here are some important ones that I call "The Four Ps of Pay Inequality."

Pay at first job. Research by Nancy Carter and Christine Silva of Catalyst, a research organization on women and business, found that pay in a first job paves the way for earnings for years to come. Many people think that businesswomen -- particularly women who hold an MBA degree -- earn less than their male counterparts because they work at lower levels of the organization or in "softer," lower paying departments (for example, human resources instead of software engineering or line management). Carter and Silva controlled for factors like these and put this misconception to rest. The surprising truth is that women were paid on average $4,600 less than men in their first post-MBA job. Then when they left this job for another and were asked, "How much were you paid in your last job?" they were, once again, offered comparably less. Lost ground is hard to make up if women are behind at the starting gate.

Parenthood penalty. When women interrupt their careers to start a family, their wages are unlikely to catch up to those workers who do not. According to Cornell University economists Francine D. Blau and Lawrence M. Kahn, the "unexplained" pay gap of nine percentage points that remains after adjusting for factors in addition to hours worked -- such as education, experience, occupation and industry -- may well be explained by time off for parenting. According to MomsRising.org, the pay gap between mothers and non-mothers is greater than that between women and men. Non-mothers endure that 9 percent gap but mothers suffer a whopping 27 percent penalty.

Part-time and other non-traditional path penalties. In the Carter and Silva research, both men and women MBAs sometimes opted for part-time schedules or work outside of traditional business employment (for example, government, non-profit, education sectors). When men resumed work full-time in the corporate sector, their salaries and career trajectories resumed, unaffected by their time off-track. But when women resumed, they advanced less than women who never left the mainstream business world and a full-time schedule. Somehow career interruptions and digressions are seen as resume-rounding for males, but derailing for women.

Penalized by plumbing. Being female is not supposed to be limiting anymore, particularly in the meritocracies of business and the bottom line, but somehow it is still. Why? When someone makes a selection or hiring decision, even among people with the same education and training, they may well find themselves favoring the one or two who are most like themselves. Research shows we tend to feel a connection to people like ourselves. So a man just might tilt toward choosing the male candidate for the job, or the promotion, or the money to start a new venture.

Case in point: Sunday's New York Times ran an extensive story on women in high-tech, profiling the experience of Candace Fleming, an industrial engineer with degrees from Harvard and Stanford and management experience at Hewlett-Packard. She is indisputably accomplished, but when she tried to raise money for her startup, a venture capitalist told her that "it didn't matter that she didn't have business cards, because all they would say was 'Mom.'"

I didn't know things like this still happened," says Ms. Fleming, 37. "But I know that, especially in risky times like the last couple years, some investors kind of retreat to investing via a template." A company owned by a woman, she adds, "is just not the standard template.

Not the standard template. When my twins were born, no one said that one was the template. They were just two babies. Equally cute, equally hungry, equally worthy. When they grow up, will the four Ps still dictate that my daughter earns less than her twin brother? Let's replace them with a fifth: Parity.

A Peaceful Revolution is a blog about innovative ideas to strengthen America's families through public policies, business practices, and cultural change. Done in collaboration withMomsRising.org, read a new post here each week.