In Cheaper, Frank and Ernestine say their Dad had that many children because of his "love of children...." This is a bit suspect, because Frank was an only child. (Lillian was the oldest of ten children.)
Lillian's biography, Making Time, has a better explanation for Frank and Lillian's decision to have that many children.
The answer was a complex mix of public and private reasons, including eugenics, women's rights, and sheer love of babies, conbined with a reluctance to discuss birth control. (Making Time, page 97)Eugenics is a key word in that quote. Eugenics, basically, is the idea that "good people should have a lot of babies" and "bad people shouldn't have any babies." That way, there will be more "good people" and fewer "bad people." The issue arises in deciding who is "good" and who is "bad."
Jodi Picoult dealt pretty clearly with the Eugenics issue in a book titled Second Glance. Eugenics was very, very popular in the early part of the 1900s. It was practiced so enthusiastically, that the state of Vermont all but wiped out the Abenaki Indian Tribe in the 1920s.
As scary as this sounds, the news is not all bad about Frank and Lillian:
Lillian and Frank believed they were great people, and had great genes, and wanted to be sure that they spread their genes around a lot!
The Gilbreths were positivie eugenicists. Rather than calling for forcible sterilization of the less "fit," they applied their theories to themselves and produced their own large family. (Making Time, page 98)