How annoying?

The doctor and her team also asked volunteers to rate different purrs – judging them based on how persistent and pleasant they seemed.

"We could then relate the scores to specific purr recordings. The key element (which made the purr less pleasant and harder to ignore) was the relative level of high-frequency sound it contained," Dr. McComb explained. "When the animal vocalizes, the vocal cords set the air in motion, and the air vibrations lock into a specific frequency." The frequency of such a sound depends on the size, length, and intensity of the vibrations.

"However, cats can also produce low-frequency purrs by activating the muscles in their vocal cords and stimulating their vibrations," explained Dr. McComb.

Because each of these sounds is produced by a different mechanism, cats can incorporate a high-frequency cry into a relaxing-sounding purr.

"How persistent and unpleasant the purring is seems to depend on the amount of energy the cat expends in producing the cry," Dr. McComb said.

Previous research has shown similarities between a cat's cry and the cry of a human baby - a sound to which humans are highly sensitive.

Dr. McComb says that this crying occurs in small amounts in normal cat purring. "However, we believe that they [cats] learn to dramatically exaggerate this crying when they see that it elicits a response from the human." He adds that this behavior seems to develop most often when there is only one cat in the human relationship.

"We obviously don't know what's going on in cats' minds. But they learn how to do it, and they do it completely intentionally," Dr. McComb said.

And what does Dr. Comb think of her cat now that she knows she was manipulated by him for so many years? "He's my inspiration for this whole study, so I forgive him—I thank him because I have something to thank him for."