College-educated women in particular underestimate the demands of parenthood and the difficulties of combining working and parenting, new research shows. An economic mystery of the last few decades has been why more women aren’t working. A new paper offers one answer: Most plan to, but are increasingly caught off guard by the time and effort it takes to raise children. The share of women in the United States labor force has leveled off since the 1990s, after steadily climbing for half a century.