But, in her fascinating new book, “The End of Men,” Hanna Rosin posits a different theory. It has to do with adaptability. Women, Rosin argues, are like immigrants who have moved to a new country. They see a new social context, and they flexibly adapt to new circumstances. Men are like immigrants who have physically moved to a new country but who have kept their minds in the old one. They speak the old language. They follow the old mores. Men are more likely to be rigid; women are more fluid.