There are many causes for this pay gap, but one factor is that women are doing work they're not getting paid for. Uncompensated labor is often referred to as "invisible work," and includes essential household responsibilities like cleaning, child care and taking care of sick family members.
According to the Organisation for Economic Co-Operation and Development, men in the United States spend 150.2 minutes a day — about 17.5 hours a week — doing unpaid labor. Women spend 243.2 minutes doing unpaid labor each day — about 28.4 hours a week. When you add both paid and unpaid work together, women still work longer hours.
The Bureau of Labor Statistics reports that Americans earn an average of $26.82 an hour. This means that if men and women were compensated for their unpaid labor, men would earn an extra $469.35 a week, and women would earn an extra $761.69 a week — which comes out to nearly $40,000 a year.