We generally assume a purring cat is a contented cat. It's safe to say a hissing cat, its ears drawn back, is not pleased.

But aside from the visage of Grumpy Cat - who may not have been grumpy at all - feline faces don't tell us much about how cats feel. Or rather, as a new study on the topic found, most of us are pretty terrible at reading cats' expressions.

Cats have a reputation for being "inscrutable", the researchers say, and their results mostly back up this notion. More than 6,000 study participants in 85 countries, the vast majority of them cat owners, watched brief cat videos and then judged the animals' moods. The average score was just under 60 percent correct - an F, if cat videos were a school subject.

However, 13 percent of participants did quite well, scoring 75 percent or above. The researchers dubbed these achievers "cat whisperers" - and said their results are important.

"Cats are telling us things with their faces, and if you're really skilled, you can spot it," said author Georgia Mason, a behavioral biologist at the University of Guelph in Ontario.

"Some people can do it - that means there's something there. That means that cats are hard to read," but not wholly inscrutable, she said.

Women, who made up three-fourths of participants, scored better than men, but not by much. Younger people did better than older people. But the most skilled diviners of feline feelings were people with professional experience involving cats, including veterinarians. (You can take a shortened version of the survey here.)

"They could be naturally brilliant, and that's why they become veterinarians," Mason said.

"But they also have a lot of opportunity to learn, and they've got a lot of motivation to learn, because they're constantly deciding: Is this cat better? Do we need to change the treatment? Does this cat need to go home? Is this cat about to take a chunk out of my throat?"

Scientists have long known that humans greatly depend on smiles, eyebrow raises, furrowed brows and other facial movements to judge how other people feel. Since a 2010 study on the grimace-like faces mice make when in pain, researchers have grown increasingly interested in understanding animal expressions, Mason said.

Several studies have focused on dogs. But Mason and her colleagues located just one peer-reviewed paper on the facial expressions of cats, despite their popularity as pets. That study focused on cats in pain.

"We wanted to know, 'Okay, do they only have pain faces?' It seems unlikely," Mason said.

Videos of cats approaching someone or getting something they wanted, such as a treat, were classified as positive. Those showing cats in pain or fleeing were deemed negative.

Easy videos - those hissing cats - were excluded. (So were any showing mating, the authors write, "due to the affectively ambiguous nature of feline mating," which can involve biting by males and other painful elements.)

The use of YouTube videos "ensures cats were behaving in cat-typical ways and gives the conclusions a sense of reality, since these are situations and expressions people may typically encounter with cats," said Kristyn R. Vitale, who researches cat behavior and cognition but was not involved in the study.

Vitale, who said she takes facial expressions into account "all the time" when interacting with cats at her Oregon State University lab, got a perfect score on a shortened online version of the new study's survey.

Mason and her colleagues say the results are valuable because people tend to be less bonded to cats than to dogs and treat them more casually. Evidence that cats make expressions that some people can detect could lead to tools that help pet owners and veterinary staff understand cats better, she said. Vitale echoed that.

The fairly poor results, including from cat owners, "indicates a large portion of people may benefit from education in cat body language and facial expression," Vitale said.

Before that happens, Mason said, she would like to answer other questions. Such as: Just what are the kitties doing with their faces that cat whisperers see - a slight eyelid twitch? A subtle widening of the eye?

"I think the cats really have these consistent facial expressions that probably they've evolved," Mason said. "People are reliably seeing something that is true and valid. But what is it?"

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