The book tells the story of time and motion study and efficiency experts Frank Bunker Gilbreth and Lillian Moller Gilbreth, and their children (said to be 11 at the start of the movie, and one baby born later, but by the end, 11 actually appear on film), as they reside in Montclair, New Jersey for many years. This fictionalized version tells the story of real-life pioneering industrial/organizational psychologist Lillian Gilbreth, her husband, and children. Lillian Gilbreth was described in the 1940s as“a genius in the art of living.”The film was based on the best-selling biographical novel that two of her twelve children wrote about their childhoods. Gilbreth’s home doubled as a sort of real-world laboratory that tested her and her husband Frank’s ideas about efficiency.
The title comes from one of Frank Sr.'s favorite jokes: it often happened that when he and his family were out driving and stopped at a red light, a pedestrian would ask, "Hey, Mister! How come you got so many kids?" Gilbreth would pretend to ponder the question carefully, and then, just as the light turned green, would say, "Well, they come cheaper by the dozen, you know," and drive off.
In real life, the Gilbreths' second eldest child, Mary, died of diphtheria at age five. The book does not explicitly explain the absence of Mary Gilbreth. It was not until the sequel, Belles on Their Toes , was published in 1950 that her death is mentioned in a footnote.