A closed stud book allows the breed to stay very pure to its type, but limits its ability to be improved. This may put a breed at a disadvantage, especially in horse breeding, where an animal is worth more if it is successful in competition even if it is not pure. It also limits the gene pool, which may make certain undesirable characteristics become accentuated in the breed, such as a poor conformational fault or a disease.
The American Kennel Club is an example of a kennel club with primarily closed books for dogs; it allows new breeds to develop under its Foundation Stock Service, but such dogs are not eligible for competition in AKC conformation shows. For the breed to move to the Miscellaneous class and then to fully recognized status, the breed's stud books must be closed.