Are you paying the women coders in your company the same as the men? Do the marketing managers who are moms get paid the same amount as the ones who are dads? How about the salespeople?

If the answer is no, I've got some bad news for you: You're probably biased. I know it's not a nice thing to say, but at this point, the evidence is squarely against you.

I understand that, if you're like most bosses, you're doing your best to be scrupulously fair. Aside from the few psychopaths out there (and this column is not for them), most entrepreneurs and managers want to do right by their employees. They want to create a positive environment where good work is rewarded and employees enjoy showing up--even virtually--each day.

But there's increasing evidence that at least some of the pay differential between men and women in the workplace--and especially that between moms and dads--is the result of preconceptions about the traditional roles of the sexes.

I should mention that in most of this research, it's not as if the guys are discriminating against women while the female managers are marvelously even-handed. Instead, in general, the women are as biased against other women as men are.

I come to this rather awkward conclusion about bias courtesy of a group of studies. Most recently, one by University of Massachusetts sociology professor Michelle Budig finds that women who have never married make about 96 percent of what men make. But married women make just 77 percent, and married mothers make 76 percent.

All else being equal, the study says, becoming a dad raises a man's income by six percent, assuming that he was already married when he became a dad. But for each child a woman has, she can expect her salary to drop by four percent.

We've known about the so-called motherhood penalty, and the related fatherhood bonus, for a while. What this newest study does, in addition to trying to discern which income levels are most affected, is to try to control for everything else that could affect how much a mom gets paid compared to a dad. In other words: It tries to come up with every reasonable explanation for why moms get paid less, and to see if the data supports it.

So do some women choose less-demanding, less well-paid jobs after having kids? Do some women have different job experiences than men, or different levels and types of education? Of course they do. And in cases where these factors come into play, some of the difference in pay between unmarried women (96 percent of a guy's paycheck) and moms (just 76 percent) can be justified. But the per-child data above takes all that into account, and the four percent drop in salary for women, per kid, and the six percent bonus for men, still stands.

Another argument might be that it's not an employer's responsibility to ensure absolute pay equity. It's the employee's responsibility to cut a good deal, and maybe men are better negotiators than women. But even this argument is steadily being undercut by research showing that women face unique hurdles at the negotiating table--hurdles that are associated with sexism and gender expectations.  

For better or worse, that leaves us to consider the role of bias in decisions about pay--and to seriously consider whether somewhere, there's a part of our brains that still thinks that a man becomes more reliable when he has kids, or that just the opposite happens with women. A guy who becomes a dad suddenly has a family to support, we figure. We don't think the same way about women who become moms, even though women are the breadwinners in 40 percent of households.

To better understand just how ingrained this prejudice is, consider that single moms pay less of a motherhood penalty, financially, than married moms. Their bosses understand that these women now have a family to support. And black men don't get a fatherhood bonus.

The exceptions are women at the very top of the income scale--those in the top ten percent. It's not clear why they get treated differently. Perhaps employers already look at those women as more akin to men in some way. Maybe it's because employers assume those moms have the resources to hire nannies, drivers, personal chefs, or whatever it takes to ensure that their lives stay fully compartmentalized. Maybe it's because those women seem to have too much to lose by reverting to traditional gender roles.

But at some point, we have to stop putting ourselves through mental contortions to come up with reasons why women somehow deserve less than men. We have to look ourselves in the mirror and ask why we manage the way we do--and if, just possibly, our judgment isn't as good as we think it is. That point is now.