Each cat has its own interests, personality, and background — all of which adds up to either wanting to be outside or not, said Dr. McMillan.

One cat owner in Portland, Ore. — who asked not to be identified to preserve her privacy from patients she works with — has tried for a year to walk her “bad boys.” She’s been able to take her cats out on leashes, but has hardly made it past her driveway without one of them running up a tree and the other wanting to go back inside.

Ollie, a well-socialized two-year-old rescue cat in Washington was happy to wander outside on the open patio of her owner, Laurel Hamers, after it had snowed. But when Ms. Hamers put a harness on her later the same day, Ollie acted as if she had no interest in going outside, and even seemed freaked out by the snow.

Cats have a reputation of being independent. They are believed to have domesticated themselves thousands of years ago, instead of being purposefully domesticated by humans. So putting them in harnesses for a walk can be especially challenging if it’s something they don’t want to do.

Some cats seem more likely to take to being on a harness, said Dr. Delgado. They are usually well socialized and able to withstand changes in their environment without making much of a fuss toward strangers. When they are very young, between two and nine weeks old, cats can most easily be socialized to be more open to new people and experiences. Cats beyond this age can still be socialized, but it may be more difficult and require more time.