Flat-faced dogs are so cute that humans cannot tell when they are sad, a study suggests.

Brachycephalic dogs, which have been bred to have a short snout and compressed skull, suffer a range of health issues and are often sickly animals as a result of their extreme form.

Scientists from Czech University of Life Sciences Prague took videos and photos of 23 dogs belonging to two breeds, either Boston terrier, a brachycephalic breed, or a jack russell terrier, a normal-headed dog.

Footage of them in four different environments — “called by name”, “play”, “separation” and “threatened by stranger” — was shown to 350 people who were asked to describe the facial expressions of the animals and what they meant.

Participants were more successful in reading the facial expressions of Boston terriers in “positive” scenarios but struggled to recognise expressions in “negative” contexts, which were more easily recognised in jack russells, data show.

The study is the first time data has shown that brachycephalic dogs have a reduced ability to communicate with facial expressions.

Study lead author Petra Eretová writes that people tend to perceive flat-faced dogs as being happy, even if they are not.

“Brachycephalic or flat-face dogs are often seen as cute, appealing and almost comically expressive, with their large eyes and short noses,” she said.

“But contrary to the stereotype, it is actually harder to read the facial expressions of flat-face dogs, compared to normal dogs.

“We found that Boston terriers were often misunderstood when expressing negative emotions, and people were in fact more likely to assume the dog was happy, even when it was scared or anxious.”

Health defects

Brachycephalic dogs, or flat-faced breeds, suffer from a range of health defects caused by extreme breeding, including breathing issues, eye infections and breeding difficulties.

Data from the Kennel Club show that flat-faced dogs in the UK are enduring their biggest decline in popularity in more than a decade, with experts cautiously optimistic the messaging about their health woes may finally be getting through to owners.

Registrations of newborn pugs, French bulldogs and English bulldogs are down more than a third compared to last year, Kennel Club statistics reveal.

The three breeds are famed for their squashed skulls and wrinkly faces and scientists have long been trying to raise awareness of their health issues.

Dr Dan O’Neill, associate professor of companion animal epidemiology at the Royal Veterinary College, told The Telegraph: “These findings help explain why so many people still refuse to accept the serious health issues linked to popular breeds such as French bulldogs, pugs and English bulldogs.

“RVC VetCompass research has paradoxically reported the French bulldog as the most owned puppy in the UK while at the same time reporting long lists of serious health issues linked to flat-faced breeds including eye, breathing, skin, sleeping and birthing problems.

“It now seems that those flat faces that make dogs look so appealing to humans despite the severe problems that they cause are also the reason owners cannot recognise the suffering.

“The flat face itself does prevent these dogs from telling us about the true welfare cost of this unnatural body shape for dogs.

“While seeing is believing, believing is not always the truth. This new study explains why we should not believe it when our eyes tell us that flat-faced dogs are always happy. Flat faces are just unable to show suffering in the way that normal dog faces can.”

The study is published in the journal Applied Animal Behaviour Science.