Something happens whenever Aaron Morrill takes his large and fluffy mutt, Donut, for her daily walk, and it’s something that always catches him a bit by surprise.

Women gather.

They flock to Donut — “a particularly cute dog,” he says — and he often finds himself surrounded by a gaggle of young women who want to know how old she is (4), if he raised her from a puppy (yes) and if they can pet her (sure).

“They see you with a dog and all their defenses go down,” said Mr. Morrill, 59, a businessman in Jersey City, N.J. “They assume you must be a decent human being. How could you have a dog and be a bad person?”

The phenomenon isn’t unique to Mr. Morrill and Donut. People with dogs are often perceived to be more approachable, happier and more empathetic, research shows. The presence of a dog can also serve as an indication a man is nurturing and capable of caregiving, said Dr. Helen Fisher, a senior research fellow at the Kinsey Institute and a chief adviser to Match.com. And that, she said, is a powerful mating signal.