The celebrity ‘trout pout’ like reality TV star Kylie Jenner really is a turn off ... after plastic surgeons devised a formula for the perfect mouth.
The lower lip should be about twice as large as the upper and make up about a tenth of the lower third of the face.
Conversely the other way round - a top lip double the size of the bottom -was deemed the ugliest.
The desired ratio was consistent with the most natural lips - before any augmentation procedure.
Taylor Swift, Angelina Jolie and Cheryl Cole are among those rated as the hottest for their natural lips.
On the other hand Meg Ryan, Melanie Griffith, Patsy Kensit and, most famously, Men Behaving Badly star Leslie Ash have been criticised for having their’s seemingly enhanced.
The researchers said Victoria’s Secret models Alessandra Ambrosio and Miranda Kerr fit the “1:2” ratio well, but their counterparts Doutzen Kroes and Rosie Huntington-Whiteley have lips that are the same, top and bottom.
Researcher Natalie Popenko explained: “It goes to show that whole faces are greater than the sum of their parts and attractiveness can also be defined by unique facial features.”
To define what makes an attractive lip in terms of volume and balance, the team used previously constructed images of an ideal white female face and adjusted the size and proportion of the upper and lower lip.
Then they sent the various options to almost 700 participants for evaluation.
The most attractive computer images based on volume and proportion were assessed to estimate the best look.
The study published in JAMA Facial Plastic Surgery said guidelines may help achieve the best outcomes in lip augmentation using injectable dermal fillers to fat grafts and implants.
Lips with a 53.5 per cent increase in surface area from the original image with a 1 to 2 ratio of upper to lower lip was voted the most attractive.
This should make up about 10 per cent of the bottom third of the face, according to the results.
Professor Brian Wong, of California University in Irvine, said: “We advocate preservation of the natural ratio or achieving a 1:2 ratio in lip augmentation procedures while avoiding the overfilled upper lip look frequently seen among celebrities.
“It’s speculative but it could be a visual cue for reproduction, good fitness and health.
“Our study isn’t about sexy, which is very hard to define. We used a simple terminology that focused on attractive. I believe that has a different meaning than sexy.
“Overall these findings were consistent with the ratio of most natural lips before any augmentation procedure.
“Our results indicate a more natural result is viewed as more aesthetically appealing which provides clinicians with an objective foundation when proceeding with augmentation.”
Lips tend to thin from around the age of three with the effects becoming more obvious from the late 30s - especially on smokers.
This has led to a clamour for swollen lips - with fuller mouths associated with youth and fertility.
Prof Wong and colleagues morphed images of 20 white 18 to 25 year-old women to generate five varied lip surfaces of each face. All, 100 were then given marks out of 10 for attractiveness by 150 people.
Four variants for each face were next created from 15 of the most attractive images before being scored by 428 volunteers.
Prof Wong said: “Well-defined and full lips convey youth and attractiveness, representing a key feature of the lower facial third.
“Whether the goal is to restore the senile lip to its previous youthful glory or mimic the pouty appearance of the social media starlet, lip augmentation has become an increasing trend.
“Although dermal fillers for lip enhancement are relatively low cost and generally safe aesthetic guidelines to direct the clinician in lip augmentation remain elusive and are primarily based on patient preference and surgeon eye.”
Added Ms Popenko: “Most people can look at a face and instinctively tell you whether that face is attractive or not, by subconsciously picking up on biologic cues like fertility, colouration, and proportions.
“As they are renowned worldwide for their facial attractiveness, I think the Victoria’s Secret models can be easily compared.”
Facial plastic surgeon Dr Catherine Winslow, of Indiana University, who reviewed the study for the journal, said it shows some celebrity lips go too far.
She said: “Led by celebrity experience and social media discussion requests are routinely made for injectables to shape the appearance of named celebrities’ lips.
“Three basic take-home points can be extracted from this study. First, there is such a thing as too much when it comes to filling the lips.
“Second, proportions and lip ratio must be respected. Third, balance of the lip region must be maintained.”
Previously, the same plastic surgeons analysed the ideal shape for a woman’s nose and found it should be slightly upturned.
They found Scarlett Johansson, The Duchess of Cambridge, Kate Beckinsale and Jessica Biel all fit the bill.