Most veterinarians in the United States recommend bitches and dogs be spayed or castrated between 6 and 9 months of age. This is not based in science; no one has performed a large-scale study in which bitches and dogs underwent gonadectomy at various ages and were tracked throughout life to determine what abnormalities developed relative to age at gonadectomy. It is thought that the current age recommendation arose after the World War II, when increasing affluence of American families first permitted them to treat animals as household pets and were, therefore, more interested in controlling manifestations of reproductive hormone secretion and very interested in making sure the animal survived surgery. Anesthetic and surgical techniques available at that time necessitated the animal be at least 6 months of age.