I call oxytocin the "moral molecule" because it motivates us to treat others with care and compassion. Oxytocin was classically associated with uterine contractions in humans, and in rodents caring for offspring. Our studies showed that a large number of agreeable human interactions—from trusting a stranger to hold money for you, to dancing, to meditating in a group—causes the release of oxytocin and, at least temporarily, makes us tangibly care about others, even complete strangers.

In our animal experiment, 100 participants came into my lab and we obtained blood samples from them to establish their baseline physiologic states. Then they went into a private room and played with a dog or cat for 15 minutes. We did a second blood draw after this, and then had participants interact with each other to see how they behaved toward humans, too. If animals caused oxytocin release in humans, it would explain my surprise attachment to my dog Teddy, and perhaps why people spend thousands of dollars to treat a pet medically rather than euthanize it and simply get a new animal.

Our previous studies showed that when humans engage in social activities with each other, oxytocin levels typically increase between 10 percent and 50 percent. The change in oxytocin, measured in blood, indexes the strength of the relationship between people. When your little daughter runs to hug you, your oxytocin could increase 100 percent. When a stranger shakes your hand, it might be 5 or 10 percent. If the stranger shaking your hand is attractive, oxytocin might increase 50 percent. Oxytocin is considered a reproductive hormone. It increases powerfully during sexual climax, establishing long-term bonds between romantic partners. Our experiments focus on what causes the brain to make oxytocin and its behavioral effects.

The dog and cat study showed that neither species consistently increased oxytocin in humans. Only 30 percent of participants had an increase in oxytocin after playing with an animal. We found that one factor predicted whether playing with a dog would increase oxytocin: the lifetime number of pets of any type one had owned.

The opposite was true for those who interacted with cats. Greater lifetime pet ownership caused oxytocin to fall linearly. Dogs are simply more "people-oriented" than cats, and previous pet ownership seems to have trained our brains to bond with them.

We also found that dogs reduced stress hormones better than cats (no surprise there!). When stress hormones were lower, people in the experiment trusted strangers with more of their own money. This may tell us why people who own dogs are judged as more trustworthy than those who don't. The human-canine bond appears to be powerful and important to both species.

Many dogs, and sometimes other mammals, exhibit another human-like behavior: play. I was curious if animals can form friendships with other animals and was invited to take part in a small-scale experiment for BBC television that would give me a chance to test this.