Even that 73 percent number is too rosy, because it only looks at a snapshot of mothers and men employed in a given year.
But mothers also spend more years out of the workforce than anyone else, usually to care for family. So the financial impact of mothers' employment patterns becomes clear only when we look across the years. The lifetime earnings of women are just 38 percent of the lifetime earnings of men.
Now that's a gap.
One of the reasons more women drop out of the workforce is because United States income tax policy is designed specifically to encourage them to drop out.