Winslow the cat, Minneapolis, 2015. Courtesy of Andy DuCett

Cats don't purr just when they're happy and content. Sometimes they purr when they're scared or in need of comfort. Some researchers think it might actually be self-healing.

But they also do it to control the weak-minded humans they live with.

All right, maybe weak-minded is too strong a term, but cats know how to take advantage of an instinct hard-wired into us.

Karen McComb, who studies animal communication at Sussex University in England, wondered why her cat's purrs in the morning were so hard for her to ignore. She calls this sound a "solicitation purr."

Hear an example of a solicitation purr at 21:29:

McComb tracked down other cats that engage in this kind of behavior and found that it's not universal. Rather, it is done by cats that have close relationships with one or two humans who pay a lot of attention to them.

She recorded the solicitation purrs and non-solicitation purrs of 10 cats and played them randomly for human test subjects. McComb asked them to rate the purrs on how urgent they were and how pleasant they were.

"It turned out that even humans with no experience with cats could do this test well," McComb said, "and found the solicitation purrs unpleasant and urgent."

To find out why humans found the sound so grating and urgent-sounding, she analyzed the solicitation purr and found something hiding within it.

"Cats were embedding within this low-frequency purr a high-frequency cry that was similar to the pitch of a baby cry," McComb said. "And probably as a result of that, [they are] able to generate this response from humans where they immediately wanted to give care."

And the reason cats can do this — basically make two sounds at once — is the way they make the purring sound. We make sounds with our vocal cords by moving air over them. But cats use their muscles.

"The purr itself is a very low-pitch sound, much lower than the cat should be able to do," McComb said. "The cat produces a pitch that is out of line with the size of its vocal cords and it can do it because it doesn't generate it by blowing air, but it twitches the vocal cords using muscles."

So cats use muscles to generate the purr sound, and air to generate the high pitch — making two sounds at once.

McComb thinks cats learn how to make this sound, designed to get us up and taking care of them, by trial and error. But she finds it remarkable that they wind up doing it in remarkably similar ways.

"In the context of living with humans, they have learned this sort of signal is effective," McComb said.

So you see, your cat is the evil genius you've suspected all along.

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