Twelve golden retrievers recently arrived in Orlando, Florida, to bring comfort to the survivors and those grieving after the mass shooting at the gay nightclub Pulse. Tim Hetzner, president of the Illinois-based K-9 Comfort Dogs team, which sent the goldens to Orlando, told The New York Times that when people who couldn’t get out of bed pet the dogs they “start smiling and in a couple cases, they started talking as much as they could.” We asked Northeastern’s Lisa Feldman Barrett, University Distinguished Professor of Psychology, to explain why the dogs had that effect and what made them helpful after this particular tragedy.

You specialize in the psychology of emotion. How can dogs bring a sense of calm in times of crisis?

Humans are a social species. What that means is that we regulate each other’s nervous systems just by being around each other, and animals—dogs, cats, my daughter’s very affectionate guinea pig—can do the same for us.

We have a variety of systems in our bodies—nervous, digestive, cardiovascular, and so on—and we have to keep them in balance to stay healthy. That state of balance is called allostasis, which means “stability through change”—chemical regulators in our bodies ebb and flow to maintain that balance. But we need help from others to do that. It’s how we’re built. For example, when you’re talking face-to-face with someone you trust, notice how you and your partner’s breathing and heartbeat synchronize, and how you start to mirror each other’s actions. There is also evidence that taking care of another, whether a pet or a person, helps regulate our nervous systems.

Why are dogs, rather than, say, cats, most frequently used as comfort animals?

Dogs are not animals that have evolved through natural selection. We’ve bred dogs to be the way they are. We selected them to have certain features that are useful to us, including very expressive eyes and, their most noted feature, unconditional love. There was an experiment in Russia that showed the effect of such breeding on red foxes. Dogs descended from wolves, but the process is the same. Over a 50-year period, the researchers bred red foxes and selected those that were the friendliest to humans to reproduce. After many generations, the researchers were caring for doglike foxes that even wagged their tails and licked their keepers. We have artificially bred other animals, but dogs are the ones that we have domesticated the most.

What made Orlando a particularly important place to bring the dogs?

Uncertainty wreaks havoc on our nervous systems, and Orlando is now the site of extreme uncertainty. No one knows whether such a massacre will happen again, and no one knows definitively why it happened: Was it a hate crime? Was it a terrorist act? Was it your almost weekly massacre that we’ve been witnessing in the United States?

Studies in hospitals, both psychiatric and nonpsychiatric, have shown many benefits to animal-assisted therapy: statistically significant reductions in anxiety, depression, pain, respiratory rate, negative mood, blood pressure, heart rate, loneliness, fatigue. They have also shown increases in perceived energy level. What is going on physiologically to enable these changes?

When I say that animals regulate our nervous systems, I mean it. Levels of hormones that kick in when we’re threatened—cortisol, epinephrine, norepinephrine—drop. We become less prepared for fight or flight, the stress response.

Dogs can bring about that regulation to a greater degree than other animals because they are fundamentally dependent. There’s a belief now that dogs misbehave when they’re left alone not because they’re trying to assert their dominance but because they’re anxious. Dogs, like toddlers, are incredibly attached and become anxious during periods of separation. That means, biologically, not only that the dog is regulating your nervous system but that you’re regulating his.