Dr. Stanley Coren, a psychology professor at the University of British Columbia, is an expert on dog-human interaction and has written extensively on the subject. His new book, The Modern Dog: A Joyful Exploration of How We Live with Dogs Today, examines interlacing canine and human histories, and how dogs can improve our quality of life. Here, Dr. Coren discusses how owning a dog can make us healthier and improve our marriage, and why puppies are better than Prozac.

Q: How and why are our relationships with dogs different from our relationships with cats and gerbils and guinea pigs and any number of other animals we keep as pets?

A: God may have created humans, but humans created dogs. For at least 14,000 years human beings have been modifying dogs. We've been doing seat-of-the-pants behaviour genetics, selecting dogs for particular qualities. In essence, the recent dogs are the perfect companions and workmates because we created them to be that way. As our technology and lifestyle changes, different breeds come in and out of existence. Right now, certainly in the Western world, dogs have more and more of a function as companion animals. We have been creating new companion animals at an incredible rate. Back in the 1950s you never would have heard of a Havanese or any of those dogs who are recognized breeds and whose job is to be sucky-face companions.

Q: How can dogs improve our quality of life?

A: Dogs improve our lives both psychologically and physically. Physically, for example, we know that exposure to a dog when you're younger actually improves the strength of your immune system. Individuals who grew up with a dog in the house when they were young tend to have fewer allergies to things like animal dander, and also tend to have fewer allergic responses in general. We believe that's simply from contact with the animal and the animal's saliva, which tends to trigger immune responses from early on.

But, much more importantly, if you pet a familiar and friendly dog, a whole bunch of changes occur in your physiology. Your heart rate slows, your breathing becomes more regular, certain stress hormones associated with the adrenal glands are reduced in your body, your muscles relax, and you show all of the signs of de-stressing. This has a whole lot of different consequences. For example, if we look at elderly individuals who would be otherwise socially isolated if not for the companionship of a dog, we find that those who have a dog require fewer medical services and are only one quarter as likely to become clinically depressed.

There's a wonderful study that was conducted by some people at the University of Pennsylvania, and they looked at men who had their first heart attack and then followed them for several years. They found out that the men who had dogs were much more likely to be alive four years later. The researchers believe that this is part of the effect of reduced stress. And the effects we're talking about are the same effects that you get with something like Prozac – only those things take anyplace between three to six weeks to build up the effects, whereas the de-stressing effect of a dog tends to occur within minutes.

Furthermore, the effect of having a dog present is stronger than having a spouse present. There's one study that looks at men in fairly stressful situations, and they could either have their dog or their spouse with them. While having the spouse with them reduced the stress some, the effects of dog was much, much greater. And part of the reason seems to be the fact that, when push comes to shove, if your spouse is unhappy with the way you're responding you're going to get criticized – and Lassie's never going to criticize you.

Q: Do you think people use dogs as a substitute for meaningful human relationships?

A: No. I think they use them to augment human relationships, and then, if nothing else is available, they can serve as a surrogate. I say augment because there's a fascinating study that came out a few years ago which looked at dog ownership and divorce rates, and they found that couples who owned a dog were less likely to divorce. The reason argued by the researchers is this. Suppose you've had a really crappy day and you really need from TLC. You come home and your spouse has had a rotten day, too. If you demand any love and support at that point in time you're going to start a fight: "My day's been just as miserable as yours; why are you doing this?" On the other hand, if you have a dog, there he is to greet you at the door saying love me, love me, love me, and you get your dose of de-stressing and now you don't need to put those demands on your spouse and the overall cumulative stress level in your relationship doesn't build up.

Q: When it comes to improving human quality of life, are there certain breeds that have greater utility than others?

A: The answer is yes and no. Generally speaking, what you need is a dog who is responsive to you. It's that social responsiveness that's important. People look at the little companion dogs, like the Cavalier King Charles and the Pug, and they say that they're frou-frou dogs that don't have any purpose. But those dogs have been specially bred to be companion dogs.

There are three types of dog intelligence. There's instinctive intelligence, which is what the dog was bred to do – so herding dogs herd and retrievers retrieve. There's adaptive intelligence, which is the relative brightness of the dog and what he can do for himself and how he can solve problems. And then there's working and obedience intelligence, which is really the equivalent to school learning – it's what dogs can learn to do for us. Every dog has instinctive intelligence, and our companion dogs have been bred to be socially empathic. And even some of the dogs that have other purposes – like some of the Spaniels and Retrievers – have been bred to have a certain empathy because they have to work in close association with people.

For example, the Span in Spaniel stands for Spain. It's not because anyone believes that these dogs developed in Spain because we know they didn't. But the reason was that, at that time, the great lovers of the time were considered to be the Spanish. So you have all of these kissy-face dogs that were named to indicate that they have a Spanish-like, Latin-loving streak. Those are the dogs that tend to give you the best support. They read your body language, they pay a lot of attention to you, and they respond socially to you by looking at you when you talk and that sort of thing. You get less of that with certain other breeds. Terriers have been designed to be work-alone dogs. They'll come up and say love me, love me, love me but you pet them twice and they'll say enough of that, I've got my own agenda. Some of the other dogs, you say snug up next to me and next thing you know you've got a lap rug to keep you company.