Throughout his life Caso wrote books about native Mesoamerican cultures, including those of the Olmec, Mixtec, Zapotec, and Aztec. He was one of the first to recognize the Olmecs as the earliest Mesoamerican civilization, declaring that they were the ""cultura madre"" (Mother culture) of Mesoamerica. His argument has subsequently been debated by Mesoamerican archaeologists; it is currently unclear how the Olmec interacted with other Mesoamerican cultures. His writing on the Zapotecs, based on his work at Monte Albán, proposed that they established hegemony over neighbouring peoples - a theory which was widely criticised at the time, but which was validated by the studies of others after Caso's death.