We acknowledge that the gender pay gap has numerous causes. But the biggest cause, as found in recent studies, is motherhood. The data shows that at age 25, college-educated women make about 90 percent as much as men. But by age 45, women make only about 55 percent as much as their male counterparts.

Thus, the gap widens for many women when they have children. The kicker: Women without children often continue to earn closer to what men do.