 Full-time working mothers with children under the age of 18 are earning 31% less than their male counterparts. That's according to some new data from Bankrate.com. Each year, mothers are missing out on roughly $17,000, according to this survey. Over 30 years, that could add up to more than a half million. A big reason why mothers are mostly affected by lost wages is they have a bigger share of household labor and caregiving responsibilities, otherwise known as the motherhood penalty.