It’s perhaps not surprising that single parents face a higher risk of living in poverty. However, a new study finds that such risk is much higher for single mothers than for single fathers, even when they both have similar jobs and education levels and work the same number of hours.

Recently published in an edition of Gender Issues, the study found that single mothers make significantly less than single fathers. In addition, single mothers seem to be financially penalized for each additional child they have, while income for single fathers remains the same or even increases with each additional child. In 2012, 28 percent of all U.S. children lived with a single parent, according to the study. Among those households, more than 4 million single mothers lived below the poverty line, while just 404,000 single fathers lived below the poverty line. The study is based on Census data between 1990 and 2010.

Study co-author Karen Kramer, an assistant professor of family studies at the University of Illinois at Urbana-Champaign, told me that when a single person is responsible for both raising children and providing the main source of income, one would think it would eliminate or at least narrow the existing gender wage gap. In other words, it seems logical that men and women would experience the same earning and economic mobility barriers if they were caring for children on their own and serving as the main income earner. But that’s not what Kramer and her colleagues found. Instead, they found that single mothers were more likely to be in poverty at “far greater rates” than single fathers, even after controlling for variables such as education and human capital, which is defined as “a resource pool of personal capabilities, talents, skills and traits that have resulted from investments in health, education, training and workforce experience.”

Study authors Kramer, Laurelle Myhra, Virginia Zuiker and Jean Bauer write:

The gender gap in income and poverty between single parents are staggering. After controlling for work characteristics, occupational characteristics, human capital (education) and demographic variables, single mothers had income from work that was 30% lower than that of single fathers in 1990 and 21% lower in 2000 and 2010. This is much larger than the estimated gender pay gap in the population. The US Department of Labor estimated that, after adjusting for differences in the attributes of men and women and to differences in human capital (work hours, occupations, number of children, education, and more) the gender pay gap is between 4.8 and 7.1 %. As such, it seems that gender plays a greater role in the wage gap between single parents than between men and women in the entire population.

In addition, conventional thinking would posit that if a single mother invests more in her education, the result would be a narrowing of the income gender gap. But again, researchers found the opposite: When both single mothers and fathers invest in their human capital in terms of occupational status and education, the income gap gets even wider. That is to say single fathers seem to enjoy greater rewards from the time and efforts they invest in employment-related skills. (Though Kramer emphasized that this finding in no way negates the importance and financial benefit of education for single mothers. Only that men seem to reap more benefits from such human capital investments.)

Overall, the study found that single mothers were much more likely to be living in the “crisis poverty” category, with 15.6 percent, 16.8 percent and 18.8 percent experiencing crisis poverty in 1990, 2000 and 2010, respectively. Single mothers were more than three times more likely to be in the crisis poverty category than single fathers across the study period. Researchers also found that single mothers worked about five hours less per week than single fathers, however their work earnings were about 60 percent less than single fathers, despite the fact that single mothers tend to have higher levels of education than single fathers.

Researchers also examined how additional children impacted earnings. They found that among single mothers, each additional child came with a decrease in earned income. Again, this wasn’t the case for single fathers, who experienced an increase in income with each new child. However, the researchers noted that while the financial benefit of additional kids among single dads remained constant, the income penalty among single moms did decrease between 1990 and 2010.

While the study didn’t pinpoint definitive reasons for the dramatic income gap, researchers did note some characteristics that could contribute. For instance, most single fathers are single as a result of divorce, whereas single mothers are more likely to have never been married. And divorce often leaves the single parent in a better financial position. Also, Kramer said, single fathers with child custody are more likely to live with a partner who can help out, while single moms are more likely to be on their own.

Kramer noted that it’s certainly not the case that all single fathers are thriving and living safely above the poverty line. Still, single mothers have a much harder time.

“Women are penalized in so many ways and men are less penalized,” Kramer told me. “Men are kind of rewarded for being a family person. Single fathers are admired — you don’t see that for single mothers.”

As with many income and wage disparities, better public policy can make a difference. For example, Kramer said, ensuring paid maternity leave, paid sick leave and raising the minimum wage could significantly impact single-mother households for the better. In addition, policies that lift single-mother households out of poverty and provide upward socioeconomic mobility could help break cycles of generational poverty and struggle.

“It’s so important for us to show how impossible it is for a single mother to raise two or three kids and have a full-time job in order to meet just the most basic needs,” Kramer said. “They are doomed to be poor if we don’t help them.”

To learn more about the study, visit Gender Issues or read this news release from the University of Illinois.

Kim Krisberg is a freelance public health writer living in Austin, Texas, and has been writing about public health for more than a decade.