This is a guest post from Cathy Reisenwitz, who blogs at Birmingham SEO Blog.

Time magazine reports that young, childless women are earning more than men. You’d be hard-pressed to find evidence of widespread discrimination against childless women in the workplace. But equally solid data confirms working mothers’ suspicions that working moms do in fact get paid less than childless women. Here’s another article on it from BusinessWeek.

Before you get up in arms about gap in pay between childless women and mothers, consider that maybe gap is fair.

Maybe moms get paid less because they work less. The majority of mothers work part time. Fully two-thirds of mothers work less than 40 hours per week, and most mothers prefer part-time over full-time. Employers pay part time workers less, whether parents or not, and offer fewer benefits because part-time workers aren’t as cost-effective for companies to employ as full-time workers. Childless women are also significantly more likely to work overtime. The vast majority of mothers, 92 percent, work less than 50 hours a week.

As a result of working less, working mothers are also less qualified than childless women. You don’t get the same amount of experience and expertise working 20 hours a week that you do working 40. Should a woman who worked full time for 2 years and then part time for 3 get the same promotion as a woman who worked full time for 5 years? A 1999 study by Klerman and Liebowitz puts it this way: “The motherhood penalty is partially explained by differences in human capital. Women with (more) children may have less experience and seniority due to the employment breaks taken to accommodate childcare.”

Meanwhile, women who get pregnant, or intend to, are more likely to choose careers that pay less.

Then there’s the issue of productivity. Do working mothers do less work during the hours they’re at work? I haven’t found any studies comparing productivity of mothers and childless women, but studies have shown female doctors are less productive than male doctors.

So if working mothers work less, are less qualified and choose lower-paying careers than childless women, should they really get paid the same? I’m thinking no.

This is a guest post from Cathy Reisenwitz, who blogs at Birmingham SEO Blog.