We hear a lot about the wage gap between men and women, but we hear less about the one between mothers and fathers. As the wage gap between single men and single women closes, sociologists have found that motherhood “is now a greater predictor of wage inequality than gender in the United States.”

With this in mind, the National Women’s Law Center created a map showing the wage gap between mothers and fathers by state, using data from the U.S. Census Bureau’s American Community Survey. The range among states is big: In Louisiana, a mother makes 58.2 cents per a dad’s dollar, while in Vermont, a mother makes 82 cents.

I called Kate Gallagher Robbins, NWLC’s director of research and policy analysis, to ask her what states with narrower disparities between moms and dads had in common. “States with smaller wage gaps, generally speaking, have higher than average minimum wages,” Robbins says. Because women are the majority of minimum wage workers, states (such as Louisiana) that pay just the federal level— $7.25 an hour—tend to have bigger gaps.

The place that has the smallest gap between mothers and fathers—just 10 cents—isn’t a state; it’s Washington, D.C. Robbins surmises that the gap is so small in D.C. because many of its workers are employed by the federal government. Government workers have “a very standard pay scale, and there’s a lot of transparency,” two factors that are related to smaller gender pay gaps, Robbins says.

Some of the states with the biggest gaps between moms and dads—including Louisiana, Wyoming, and West Virginia—are ones where many of the available jobs are concentrated in the energy industry. “Those are types of jobs where there’s a lot of occupational segregation, and they pay reasonably well, even if you don’t have a degree,” Robbins explains. So a woman without a high school education in Louisiana might be a waitress making a tipped employee minimum wage of $2.13 an hour, while her male counterpart is working on a oil rig, making a great deal more.

One surprising thing about NWLC’s map is that paid parental leave is not necessarily correlated with a small wage gap between mothers and fathers, even though mothers are more likely to stay in the workforce when they get paid leave. Take California and New Jersey, two of the three states in the Union with state-mandated paid leave: While California moms make 76.4 cents per a dad’s dollar, New Jersey moms make just 66.7 cents—putting it in the bottom 20. That shows that the maternal wage gap is a knotty, complicated problem and that we’ll probably need several different kinds of solutions to close it.