Indeed, several studies have measured the beneficial effects animals can have on people. In 1985, University of Pennsylvania researchers reported that stroking a dog or cat can lower blood pressure in individuals with hypertension.

Another study found that elderly people caring for a pet showed improved alertness. Dog owners who have suffered a heart attack have been found to be significantly less likely to die in the year following the heart attack compared to heart attack survivors who didn't own dogs.

One multi-state study found that medication costs dropped an average of $3.80 per patient, per day, in new nursing home facilities where animals were included in the environment.

Even fish can help people feel better. One study found that patients experienced less anxiety prior to undergoing a medical procedure if they had been watching fish swimming in an aquarium beforehand.

Such research has helped to confirm what some healthcare providers have sensed for centuries. In 1790s England, the Society of Friends ran a well-documented retreat for the mentally ill where patients learned to care for animals and work in a garden as part of their therapy.

A similar program for people with disabilities has existed in rural Gheel, Belgium since the ninth century, according to Mary Burch, Ph.D., author of Volunteering With Your Pet.

In the United States, animal visitation has been used in a mental health program at St. Elizabeth's Hospital in Washington, D.C. since 1919. During World War II, patients at Army Air Corps hospitals were encouraged by Red Cross aides to do farm work to keep their minds off the war.

A decade later, New York child psychiatrist Borris Levinson, Ph.D. noted that when his dog Jingles attended therapy sessions, he was able to make more progress with one disturbed patient.

Levinson began using his dog in sessions with other patients and found that many of the children who were usually withdrawn would interact more willingly with Jingles. His research began a trend toward standardizing animal-assisted therapy techniques for both the human and animal participants.

Rigorous Training, Rewarding Work
Almost any domesticated animal can become a therapy animal. There is even experimental work using dolphins in therapy with disabled children and adults. Among the Delta Society's registered "Pet Partners" are rabbits, guinea pigs, cats, goats, pot-bellied pigs, horses, donkeys, African gray parrots, cockatoos, chickens, and llamas. But dogs are still by far the most common and popular animal therapists.