Recently, my son mentioned that he’s decided to ask for a raise. My immediate (but unspoken) reaction was caution: Don’t rock the boat. Look what a great learning opportunity you have. In this economy just be grateful to have such a good job. That mindset exemplifies a gender difference—women often are afraid to ask for a raise or insist on a promotion.

When I think about my son’s situation more objectively, I realize he’s right. For the last nine months, he has been doing a product manager’s job without the official title or the additional money a product manager would make. Higher-ups in the organization have publicly recognized his efforts, so it is a good time to ask for a raise.

I was surprised to realize how ingrained my caution is. Because I’m aware of women’s tendency to be self-effacing, I thought that mindset no longer had power over me. And yet, I can recall times that I’ve devalued my contributions. I know that women worry about being disliked if they advocate for themselves—I’ve done that, too.

Reluctance to rock the boat is one of the reasons why women’s salaries continue to lag behind men’s.

The pay gap figure that’s often used is that the median earnings of full-time female workers are 77 percent of the median earnings of full-time male workers (Bureau of Labor Statistics).

Often women aren’t confident, effective negotiators. Many times women feel grateful for the job—lucky to have it at all—instead of recognizing the value we contribute. Or women want to be liked and worry that pushing for what we’re due will be seen as being aggressive. There are good reasons for women’s concerns.

A recent New York Times article describes research that validates the persistent, and often unconscious, perception that women who ask for raises and promotions are unfeminine and demanding.

Linda C. Babcock, one of the researchers the article cites, addresses the “apples to apples” argument in her book Women Don’t Ask. When comparing the salaries and negotiating experiences of single men and women who had just earned MBAs, she asked, “When you got your offer, did you attempt to negotiate?” She found that about 7 percent of women attempted to negotiate, while 57 percent of men did. Of those people who negotiated, they were able to increase their salary by over 7 percent.

There are other systemic reasons for the wage earnings gap. Here are some of the common counter arguments rationalizing it:

Women often take more time off – They are more likely to interrupt their careers when they become parents, and they are more likely to be the default caregivers for sick children and parents.

Although I couldn’t find evidence to conclusively confirm or disprove it, this assertion feels true. The trend is certainly true of the women I know. Obviously, if a woman works fewer hours and/or her family leave is unpaid, she will make less money in a year when she has heavy caretaking responsibilities (whether or not she should be responsible for more of the caretaking is a different issue).

But it doesn’t make sense for women to be penalized long-term for shouldering that responsibility. For example, 10 years’ experience should be 10 years’ experience, whether the employee is a man or a woman. If a woman works full-time for five years, then takes off for five years to be with her children, then brings her skills up to speed and returns to full-time work for an additional five years, her pay should be that of a person who has 10 years’ experience. The interruption shouldn’t have a lasting effect, but often it does.

Women often enter lower paying professions such as teaching, nursing and food preparation, and that’s why they earn less.

The low-paying profession argument deserves a closer look. Female elementary school teachers make 90.9 percent of what men make and female nurses make 85.6 percent of what their male counterparts make (Institute for Women’s Policy Research IWPR #C350a). That’s still a wage gap.

Women in high-paying industries also lag behind men. Female physicians and surgeons earn 71 percent of what males in those fields earn. Female lawyers earn 77.1 percent of what male lawyers make.

The size of the gap may vary, but the fact of a persistent wage gap is undeniable.

The gap grows during the span of a woman’s career. If a woman doesn’t make the same salary as a man at the beginning of her career, she is very likely to be playing catch-up at her next job. The lag will compound over the course of her career.

What makes pay equity even more challenging is that employees don’t know the salary range for their positions. Many employers have spoken or unspoken rules that forbid inquiring about coworkers’ salaries, so employees can’t ask without fear of retaliation. The Paycheck Fairness Act 2014 is intended to make it easier for women to know what their counterparts are being paid and give women the data as well as the legal clout to insist on equal pay. But even if salary transparency were legal, discussing income is nearly taboo in our culture.

As history and other anti-discrimination laws have taught us, changing hearts, minds and cultures is even harder than changing laws. Addressing women’s reluctance to negotiate and employers’ subtle bias against women who do seek raises and promotions are the real challenges.