Final answer:
The motherhood penalty is the factor that accounts for the greatest share of the difference in the earnings of working women and men in the United States.
Step-by-step explanation:
The factor that accounts for the greatest share of the difference in the earnings of working women and men in the United States is the motherhood penalty. Women are more likely to bear a larger share of household responsibilities, such as taking care of young children, which can result in dropping out of the labor force for several years or working on a reduced schedule. This reduced job experience can contribute to lower average wages for women compared to men.