Dogs have been selectively bred for thousands of years, sometimes by inbreeding dogs from the same ancestral lines, sometimes by mixing dogs from very different lines. The process continues today, resulting in a wide variety of breeds, hybrids, and types of dog. Standard or Performance types are generally more athletic with longer muzzles and a more square head. They have also won the best in show title at the Westminster Kennel Club Dog Show on four occasions, and have been linked to the President of the United States on several occasions, with owners including Richard Nixon and Harry S. Breeds are usually categorized by the functional type from which the breed was developed. In 2013, the cocker spaniel ranked 29th the American Kennel Club registration statistics of historical comparisons and notable trends.
It is estimated that three-quarters of the world's dog population lives in the developing world as feral, village or community dogs, with pet dogs uncommon. "The most widespread form of interspecies bonding occurs between humans and dogs" and the keeping of dogs as companions, particularly by elites, has a long history. (As a possible example, at the Natufian culture site of Ain Mallaha in Israel, dated to 12,000 BC, the remains of an elderly human and a four-to-five-month-old puppy were found buried together). However, pet dog populations grew significantly after World War II as suburbanization increased. In the 1950s and 1960s, dogs were kept outside more often than they tend to be today (using the expression "in the doghouse" to describe exclusion from the group signifies the distance between the doghouse and the home) and were still primarily functional, acting as a guard, children's playmate, or walking companion. Early training and socialization both in the home and outside of the home is essential for this breed. The American Hairless Terrier does not have dental issues (absent premolars) or other characteristics associated with the dominant hairless gene. Breeds are usually categorized by the functional type from which the breed was developed. These dogs remained in the Brooke family for nearly 300 years.